Maybe a time before Christmas by Yorkshire river (12)
I believe the answer is:
peradventure
'maybe' is the definition.
(synonyms)
'a time before christmas by yorkshire river' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'time before christmas' becomes 'advent' (advent is the period leading up to Christmas Day).
'by' says to put letters next to each other.
'yorkshire river' becomes 'ure' (River Ure).
'per'+'advent'+'ure'='PERADVENTURE'
(Other definitions for peradventure that I've seen before include "by chance" , "Possibly (archaic)" , "Maybe" , "perhaps" .)