Tavener's first work -- score is superb (3-5)
I believe the answer is:
top-notch
'superb' is the definition.
(I've seen this before)
'tavener's first work score' is the subsidiary indication.
'first' says to take the initial letters.
'work' becomes 'op' (common abbreviation for 'opus').
'score' becomes 'notch' (I've seen this before**).
The initial letter of 'tavener' is 't'.
't'+'op'+'notch'='topnotch'
'is' is the link.
(Other definitions for top-notch that I've seen before include "First-rate" , "Excellent (informal)" , "Wonderful" , "Grade 1" , "Superb" .)