Sow hit hard on the head must become dizzy? (14)
I believe the answer is:
scatterbrained
'dizzy?' is the definition.
(thesaurus)
'sow hit hard on the head' is the wordplay.
'sow' becomes 'scatter' (scattering is a kind of sowing).
'hit hard on the head' becomes 'brained' (brain can mean to hit on the head).
'scatter'+'brained'='SCATTERBRAINED'
'must become' acts as a link.
(Other definitions for scatterbrained that I've seen before include "Inattentive" , "Unfocused" , "Inclined to forget" , "easily distracted - with 20d" , "absent-minded" .)