Singer reversed test, having ultimately scant appeal (6)
I believe the answer is:
tomtit
'singer' is the definition.
(tomtit is a songbird)
'reversed test having ultimately scant appeal' is the wordplay.
'reversed' shows that the letters should be reversed in order.
'test' becomes 'MOT' (as in an MOT test for a vehicle).
'having' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
'appeal' becomes 'it'.
The final letter of 'scant' is 't'.
'mot' backwards is 'tom'.
'tom'+'t'+'it'='TOMTIT'
(Other definitions for tomtit that I've seen before include "bird - with 25d" , "Small bird" , "another bird gets both" , "Popular name for a songbird" , "One of various small common birds" .)