In first half of last month I would settle (6)
I believe the answer is:
decide
'settle' is the definition.
(I know that settle can be written as decide)
'in first half of last month i would' is the wordplay.
'in' is an insertion indicator.
'first half of' means to take half the letters.
'last month' becomes 'december' (last month of the year).
'i would' becomes 'I'd'.
'december' with half the letters taken is 'dece'.
'dece' going around 'id' is 'DECIDE'.
(Other definitions for decide that I've seen before include "Reach a judgment" , "Make up one's mind" , "Resolve; give judgement" , "Settle conclusively" , "Resolve or give a judgment" .)