Substitute fabric left with expert (7)
I believe the answer is:
replace
'substitute' is the definition.
('replace' can be a synonym of 'substitute')
'fabric left with expert' is the wordplay.
'fabric' becomes 'rep' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'with' means one lot of letters go next to another.
'expert' becomes 'ace' (ace is a kind of expert**).
'rep'+'l'+'ace'='REPLACE'
(Other definitions for replace that I've seen before include "Substitute, supplant" , "Put back or substitute" , "Substitute for (someone)" , "succeed" , "Put something back where it belongs" .)