Flag bike down at last with gesture (6)
I believe the answer is:
ensign
'flag' is the definition.
(synonyms)
'bike down at last with gesture' is the wordplay.
'at last' suggests the final letters.
'with' means one lot of letters go next to another.
'gesture' becomes 'sign' (signing is a kind of gesturing).
The last letters of 'bike down' are 'en'.
'en'+'sign'='ENSIGN'
(Other definitions for ensign that I've seen before include "Flag indicating allegiance" , "Flag; standard-bearer" , "Badge, flag" , "Soldier; standard" , "Small flag or junior naval officer" .)