Half a tenner (5)
I believe the answer is:
fiver
'half a tenner' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fiver that I've seen before include "Smallest English note" , "Hazel's brother in Watership Down" , "Where to see portrait of Elizabeth Fry" , "Bank note" , "Banknote (informal)" .)