Function under spirit's strong effect (6)
I believe the answer is:
impact
'strong effect' is the definition.
(I've seen this before)
'function under spirit's' is the wordplay.
'function' becomes 'act' (I've seen this before).
'under' says to put letters next to each other.
'spirit' becomes 'imp' (I have seen 'Evil spirit ' mean 'imp' so perhaps 'spirit' could also mean 'imp').
'act' put after 'imp' is 'IMPACT'.
(Other definitions for impact that I've seen before include "Repercussion" , "Forceful contact between two objects" , "hit, in a big way" , "Forceful striking" , "Bear upon" .)