Indulge sailor visiting Brittany on vacation (4)
I believe the answer is:
baby
'indulge' is the definition.
(synonyms)
'sailor visiting brittany on vacation' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'visiting' means one lot of letters goes inside another.
'on vacation' suggests removing the centre.
'brittany' with its middle taken out is 'by'.
'ab' placed within 'by' is 'BABY'.
(Other definitions for baby that I've seen before include "Very young child" , "Timid person; child" , "Honey" , "Lumbered, left holding this" , "needing dummy?" .)