Tot by sea in Brittany in holiday season (6)
I believe the answer is:
summer
'holiday season' is the definition.
'tot by sea in brittany' is the wordplay.
'tot' becomes 'sum' ('sum' can be a synonym of 'tot').
'by' means one lot of letters go next to another.
'sea in brittany' becomes 'mer' (French for 'sea').
'sum'+'mer'='SUMMER'
'in' acts as a link.
(Other definitions for summer that I've seen before include "'Now is the winter of our discontent / Made glorious ... by this sun of York' (Richard III)" , "'Shall I compare thee to a . . . . . .'s day?' (Shakespeare)" , "peak period?" , "Best of times" , "'. . . . . . time an' the livin' is easy' (song)" .)