Fit and well — antismoker at heart (5)
I believe the answer is:
spasm
'fit' is the definition.
(thesaurus)
'well antismoker at heart' is the wordplay.
'well' becomes 'spa' (I've seen this before).
'at heart' says to take the centre.
The middle letters of 'antismoker' are 'sm'.
'spa'+'sm'='SPASM'
'and' acts as a link.
(Other definitions for spasm that I've seen before include "Sudden convulsion" , "Fit" , "Sudden muscular contraction" , "Involuntary muscular contraction" , "sudden attack" .)