Boy's brother associated with the Spanish tambourine (7)
I believe the answer is:
timbrel
'tambourine' is the definition.
(thesaurus)
'boy's brother associated with the spanish' is the wordplay.
'boy' becomes 'tim' (**).
'brother' becomes 'br'.
'associated with' means one lot of letters go next to another.
'the spanish' becomes 'el'.
'tim'+'br'+'el'='TIMBREL'
(Other definitions for timbrel that I've seen before include "old instrument" , "Biblical percussion instrument" , "Hand drum" .)