With sick mind (4)
I believe the answer is:
will
'mind' is the definition.
(I've seen this before)
'with sick' is the wordplay.
'with' becomes 'w' (abbreviation).
'sick' becomes 'ill' (synonyms).
'w'+'ill'='WILL'
(Other definitions for will that I've seen before include "mind" , "Fixed intention" , "Shakespeare < 's intention to leave something" , "Resolve - testament" , "Volition" .)