Pile into it though it's unsuitable (5)
I believe the answer is:
inapt
'it's unsuitable' is the definition.
(I've seen this before)
'pile into it' is the wordplay.
'pile' becomes 'nap' (both can refer to the surface of a fabric).
'into' indicates putting letters inside.
'nap' placed within 'it' is 'INAPT'.
'though' is the link.
(Other definitions for inapt that I've seen before include "Free from deceit" , "Out of place" , "Ill-suited" , "not fit for purpose" , "Improper" .)