Gathers best cello concerto, to an extent, is uplifting (8)
I believe the answer is:
collects
'gathers' is the definition.
('collect' can be a synonym of 'gather')
'best cello concerto to an extent is uplifting' is the wordplay.
'to an extent' indicates a hidden word.
'is uplifting' shows that the letters should be reversed in order.
'stcelloc' is hidden within 'best cello concerto'.
'stcelloc' backwards is 'COLLECTS'.
(Other definitions for collects that I've seen before include "Gets or gathers together" , "Accumulates, collates" , "prayers" , "Assembles" .)