Player succeeded in that specific instrument, mostly (8)
I believe the answer is:
thespian
'player' is the definition.
(I know that player can be written as thespian)
'succeeded in that specific instrument mostly' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'in' is an insertion indicator.
'that specific' becomes 'the' ('the' is used to refer to a specific item).
'instrument mostly' becomes 'pian' (I can't explain this - if you can you should believe this answer much more).
'the'+'pian'='thepian'
's' placed inside 'thepian' is 'THESPIAN'.
Can you help me to learn more?
(Other definitions for thespian that I've seen before include "Relating to the theatre" , "Player" , "Tragic actor" , "Theatrical type" , "Stage-player" .)