Left by pier, bananas past sell-by date? (8)
I believe the answer is:
overripe
'past sell-by date?' is the definition.
'left by pier bananas' is the wordplay.
'left' becomes 'over' (I've seen this before**).
'by' means one lot of letters go next to another.
'bananas' indicates anagramming the letters (informal term for mad).
'pier' with letters rearranged gives 'ripe'.
'over'+'ripe'='OVERRIPE'
(Other definitions for overripe that I've seen before include "I reprove for being too mature" , "More than ready" , "(Of fruit, cheese, etc) past the usual stage for eating" , "(Of fruit) past its best" , "Matured too long , not good to eat" .)