Ignore number in boat (4)
I believe the answer is:
snub
'ignore' is the definition.
(synonyms)
'number in boat' is the wordplay.
'number' becomes 'n' ().
'in' indicates putting letters inside.
'boat' becomes 'sub' (I've seen this before).
'n' put inside 'sub' is 'SNUB'.
(Other definitions for snub that I've seen before include "Ignore, spurn disdainfully" , "Rebuff or reject someone" , "Refuse to acknowledge" , "Treat with disdain" , "Reject a person with cold behaviour" .)