Distribute headgear around Maine, to a certain extent (8)
I believe the answer is:
'to a certain extent' is the definition.
(I've seen this before)
'distribute headgear around maine' is the wordplay.
'distribute' becomes 'sow' (sowing is a kind of distributing).
'headgear' becomes 'hat' (hat is a kind of headgear).
'around' is an insertion indicator.
'maine' becomes 'me'.
'sowhat' enclosing 'me' is 'SOMEWHAT'.
(Other definitions for somewhat that I've seen before include "To some degree, sort of" , "Quite" , "Rather, a little" , "To a certain degree - a bit" , "Moderately" .)