Medicine for heavyweight, I see (5)
I believe the answer is:
tonic
'medicine' is the definition.
(I know that tonic is a type of medicine)
'heavyweight i see' is the wordplay.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'see' becomes 'C' (the word for the letter, according to Chambers).
'ton'+'i'+'c'='TONIC'
'for' is the link.
(Other definitions for tonic that I've seen before include "Fillip" , "makes you feel good?" , "Stimulant" , "Stimulus" , "that may be mixed with gin" .)