Such underwear may reveal more than you think! (8,4)
I believe the answer is:
freudian slip
'reveal more than you think' is the definition.
I can't judge whether this definition defines the answer.
'such underwear' is the wordplay.
'such' becomes 'freudian' (I can't explain this - if you can you should give a lot more credence to this answer).
'underwear' becomes 'slip' (I've seen this before).
'freudian'+'slip'='FREUDIAN SLIP'
'may' acts as a link.
Can you help me to learn more?
(Other definitions for freudian slip that I've seen before include "Words revealing unconscious thoughts" , "Mistake revealing hidden concerns" , "Spoken error revealing unconscious thought" , "Error betraying inner thoughts" , "It betrays unconscious desires" .)