Marry sweetheart before 1 November (4)
I believe the answer is:
join
'marry' is the definition.
(thesaurus)
'sweetheart before 1 november' is the wordplay.
'sweetheart' becomes 'jo' (Scottish term for darling or sweetheart).
'before' says to put letters next to each other.
'i' becomes 'i' (roman numeral).
'november' becomes 'N' (phonetic alphabet: alpha, bravo, charlie etc.).
'jo'+'i'+'n'='JOIN'
(Other definitions for join that I've seen before include "Associate, combine" , "Enrol in" , "Amalgamate" , "make ends meet" , "Attach" .)