Male setter's inside playing Arab instrument (7)
I believe the answer is:
marimba
'instrument' is the definition.
(marimba is a kind of instrument)
'male setter's inside playing arab' is the wordplay.
'male' becomes 'm' (common abbreviation).
'setter's' becomes 'im' (I've seen this in other clues**).
'inside' is an insertion indicator.
'playing' indicates an anagram.
'arab' is an anagram of 'arba'.
'im' put into 'arba' is 'arimba'.
'm'+'arimba'='MARIMBA'
(Other definitions for marimba that I've seen before include "Form of xylophone" , "Latin American instrument like a xylophone" , "Xylophone-like instr." , "Jazz instrument" , "Deep toned percussion instrument" .)