As with the other achievements, this one is buggy. Some people have unlocked this naturally but I did not. I'd recommend saving at least 100,000 ETO over a game, going for this achievement last (you'll probably earn around 200,000 in a game from missions and collecting ETO across the map, and working towards other achievements) and spending it all in one visit at one shop using one Operative.
The shop I'd suggest using - because it worked for me - is called 'FYDEL' on Regent Street (the west-most shop on Regent Street, which is near the North West point of City of Westminster) which had just over 100,000 ETO in value across the clothing categories.
Other shops may work fine too - 'Ferbland' next door has just over 100,000 ETO in value across their categories too, assuming Earrings count.
I'd advise against using an operative that has a discount or recruiting an operative that buys clothes randomly, as who knows how that will affect your achievement. I actually spent just under 100,000 when this popped - the trigger is a mystery.