Feeling unwell, perhaps? Not half! (3,3)
I believe the answer is:
ill say
'not half' is the definition.
(both can be used to express agreement)
'feeling unwell perhaps?' is the wordplay.
'feeling unwell' becomes 'i'll' (ill means unwell or sick).
'perhaps?' becomes 'say' (both can mean 'for example').
'ill'+'say'='ILL SAY'
(Other definitions for ill say that I've seen before include "Indubitably" , "expression of agreement" , "I agree wholeheartedly!" , "Rather!" , "Injuries" .)