Stern seller sent bananas (10)
I believe the answer is:
relentless
'stern' is the definition.
(synonyms)
'seller sent bananas' is the wordplay.
'bananas' indicates anagramming the letters (bananas can informally mean crazy).
'seller'+'sent'='sellersent'
'sellersent' is an anagram of 'RELENTLESS'.
(Other definitions for relentless that I've seen before include "Harsh, inflexible" , "Unyielding - implacable" , "Allowing no respite" , "Oppressively constant; inflexible" , "sustained" .)