Fit in extensive jacket of brocade, at first (6)
I believe the answer is:
belong
'fit in' is the definition.
(to belong somewhere is to fit in there)
'extensive jacket of brocade at first' is the wordplay.
'extensive' becomes 'long-' (I've seen this in another clue).
'jacket of' says to hollow out the word (remove centre letters) (only the word's outer letters or 'jacket' remain).
'at first' is a charade indicator (letters next to each other).
'brocade' with its middle taken out is 'be'.
'long' after 'be' is 'BELONG'.
(Other definitions for belong that I've seen before include "Pertain" , "accepted socially" , "to have a home" , "feel like one of the family" , "find a place?" .)