When describing supporter this compiler's harsh (8)
I believe the answer is:
abrasive
'harsh' is the definition.
(I've seen this before)
'when describing supporter this compiler's' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'describing' indicates putting letters inside (describe can mean to form the outline of).
'supporter' becomes 'bra' (a bra gives support to breasts).
'this compiler's' becomes 'i've' ('I have' referring to the crossword setter).
'as' enclosing 'bra' is 'abras'.
'abras'+'ive'='ABRASIVE'
(Other definitions for abrasive that I've seen before include "smooth? The opposite" , "Caustic, harsh" , "Harsh; eg, emery" , "Harsh - chafing" , "Rough and scratchy" .)