Shrink undergarment? That offers revealing glimpse! (8,4)
I believe the answer is:
freudian slip
'that offers revealing glimpse' is the definition.
(a Freudian slip reveals something subconscious)
'shrink undergarment?' is the wordplay.
'shrink' becomes 'freudian' (I am not sure about this - if you are sure you should believe this answer much more).
'undergarment?' becomes 'slip' (slip is a kind of undergarment).
'freudian'+'slip'='FREUDIAN SLIP'
Can you help me to learn more?
(Other definitions for freudian slip that I've seen before include "Unintentional error" , "Error betraying inner thoughts" , "Words revealing unconscious thoughts" , "Slip of the tongue, revealing unconscious thought" , "can be a revealing thing" .)