On retiring, skinny front of garment reveals zip (7)
I believe the answer is:
nothing
'zip' is the definition.
('nothing' can be a synonym of 'zip')
'on retiring skinny front of garment' is the wordplay.
'retiring' says the letters should be written in reverse.
'skinny' becomes 'thin' ('thin' can be similar in meaning to 'skinny').
'front of' indicates taking the first letters.
The initial letter of 'garment' is 'g'.
'on' back-to-front is 'no'.
'no'+'thin'+'g'='NOTHING'
'reveals' acts as a link.
(Other definitions for nothing that I've seen before include "what House of Peers did particularly well?" , "Nought; of no value" , "Zilch, nada" , "trivial event" , "See 1" .)