Instruction to musicians playing trio to hold melody back? (8)
I believe the answer is:
ritenuto
'instruction to musicians' is the definition.
(musical instruction to reduce speed)
'playing trio to hold melody back?' is the wordplay.
'playing' indicates an anagram.
'to hold' is an insertion indicator.
'melody' becomes 'tune' (synonyms).
'back?' is a reversal indicator (I've seen 'backed' mean this).
'tune' in reverse letter order is 'enut'.
'trio' anagrammed gives 'rito'.
'rito' enclosing 'enut' is 'RITENUTO'.
(Other definitions for ritenuto that I've seen before include "slowing immediately" , "coming to a sudden slowdown" , "Immediately reducing in speed (music)" .)