A pair of black cotton trousers fits (7)
I believe the answer is:
accords
'fits' is the definition.
(according is a kind of fitting)
'a pair of black cotton trousers' is the wordplay.
'a pair of black cotton' becomes 'ac' (I can't justify this - if you can you should believe this answer much more).
'trousers' becomes 'cords' (cords is a kind of trousers).
'ac'+'cords'='ACCORDS'
Can you help me to learn more?
(Other definitions for accords that I've seen before include "Agreements between two states" , "is in agreement" , "Is in harmony" .)