Chemist's fast taking ill (8)
I believe the answer is:
pharmacy
'chemist's' is the definition.
(I've seen this before)
'fast taking ill' is the wordplay.
'fast' becomes 'pacy' (I've seen this before).
'taking' is an insertion indicator.
'ill' becomes 'harm' (as in doing people ill).
'pacy' placed around 'harm' is 'PHARMACY'.
(Other definitions for pharmacy that I've seen before include "Dispensary" , "drug dealer" , "It dispenses medicines" , "Medical establishment" , "where a remedy is sought" .)