Pester the Spanish to return instrument (5)
I believe the answer is:
bugle
'instrument' is the definition.
(bugle is a kind of instrument)
'pester the spanish to return' is the wordplay.
'pester' becomes 'bug' ('bug' can be a synonym of 'pester').
'the spanish' becomes 'el'.
'to return' shows that the letters should be reversed in order.
'el' reversed gives 'le'.
'bug'+'le'='BUGLE'
(Other definitions for bugle that I've seen before include "Trumpet-like instrument" , "A military trumpet" , "Creeping plant with blue flowers" , "It's used for military calls" , "costume ornament?" .)