Study hint about day to decide (8)
I believe the answer is:
conclude
'decide' is the definition.
(I've seen this before)
'study hint about day' is the wordplay.
'study' becomes 'con' (con can mean to study).
'hint' becomes 'clue' (synonyms).
'about' means one lot of letters goes inside another.
'day' becomes 'd' (abbreviation for day).
'clue' going around 'd' is 'clude'.
'con'+'clude'='CONCLUDE'
'to' is the link.
(Other definitions for conclude that I've seen before include "Finally decide" , "Close, end" , "Decide - terminate" , "Terminate; deduce" , "Finish - deduce" .)