Powerful tenor ends in tune (6)
I believe the answer is:
strong
'powerful' is the definition.
('strong' is associated in meaning with 'powerful')
'tenor ends in tune' is the wordplay.
'ends' means to remove the middle letters.
'in' is an insertion indicator.
'tune' becomes 'song' (I've seen this before**).
'tenor' with its middle removed is 'tr'.
'tr' placed inside 'song' is 'STRONG'.
(Other definitions for strong that I've seen before include "Able to withstand attack" , "Intense" , "Vigorous" , "strong" , "Not faint or feeble" .)