Examine top of posh gown (5)
I believe the answer is:
probe
'examine' is the definition.
('probe' can be a synonym of 'examine')
'top of posh gown' is the wordplay.
'top of' suggests taking the first letters.
'gown' becomes 'robe' ('robe' can be a synonym of 'gown').
The first letter of 'posh' is 'p'.
'p'+'robe'='PROBE'
(Other definitions for probe that I've seen before include "sound" , "Examine searchingly" , "Poke into" , "Question or examine closely" , "Explore, examine" .)