Bear on a Piano by surreal painter (9)
I believe the answer is:
appertain
'bear on' is the definition.
(I've seen this before)
'a piano by surreal painter' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'by' means one lot of letters go next to another.
'surreal' is an anagram indicator (I've seen this in other clues).
'painter' with letters rearranged gives 'pertain'.
'a'+'p'+'pertain'='APPERTAIN'
(Other definitions for appertain that I've seen before include "Belong, relate (to)" , "Be relevant to" , "Bear on" , "Concern" .)