Now and then sell one fruit (4)
I believe the answer is:
sloe
'fruit' is the definition.
(I've seen this before)
'now and then sell one' is the wordplay.
'now and then' indicates alternate letters (letters taken on a regular basis, now and then).
'sell'+'one'='sellone'
The alternating letters of 'sellone' are 'SLOE'.
(Other definitions for sloe that I've seen before include "Fruit of the blackthorn does't sound fast" , "Musical study" , "Fruit often used for gin" , "Blackthorn, used in making a type of gin" , "Wild plum" .)