Colour of banana, next to sink, is a warning sign (6,4)
I believe the answer is:
yellow flag
'warning sign' is the definition.
(hazard flag in motor racing)
'colour of banana next to sink' is the wordplay.
'colour of banana' becomes 'yellow' (bananas are yellow).
'next to' says to put letters next to each other.
'sink' becomes 'flag' (flagging is a kind of sinking).
'yellow'+'flag'='YELLOW FLAG'
'is a' is the link.
(Other definitions for yellow flag that I've seen before include "Signal letter Q, flown to indicate ship is carrying disease" , "Signal of quarantined ship - wild iris" .)