Fibre replaced Lycra in crop tops (7)
I believe the answer is:
acrylic
'fibre' is the definition.
(acrylic is a kind of fibre)
'replaced lycra in crop tops' is the wordplay.
'replaced' indicates anagramming the letters.
'tops' indicates taking the first letters.
The first letters of 'in crop' is 'ic'.
'lycra' anagrammed gives 'acryl'.
'acryl'+'ic'='ACRYLIC'
(Other definitions for acrylic that I've seen before include "Pungent acid" , "Synthetic fibre or paint" , "Type of hard paint" , "Textile fibre - icy Carl (anag)" , "Kind of paint" .)