Seamstress at first makes fun of garments (6)
I believe the answer is:
smocks
'garments' is the definition.
(I've seen this before)
'seamstress at first makes fun of' is the wordplay.
'at first' indicates taking the first letters.
'makes fun of' becomes 'mocks' (I've seen this in another clue).
The initial letter of 'seamstress' is 's'.
's'+'mocks'='SMOCKS'
(Other definitions for smocks that I've seen before include "Loose long coveralls" , "dresses" , "countryfolks' garb?" , "Loose coveralls, worn by artists perhaps" , "artist's clothes" .)