Smother the Spanish attacks in retreat (6)
I believe the answer is:
stifle
'smother' is the definition.
('stifle' can be a synonym of 'smother')
'the spanish attacks in retreat' is the wordplay.
'the spanish' becomes 'el'.
'attacks' becomes 'fits' (fit is a kind of attack).
'in retreat' is a reversal indicator.
'el'+'fits'='elfits'
'elfits' reversed gives 'STIFLE'.
(Other definitions for stifle that I've seen before include "Have difficulty breathing" , "Smother, check" , "Suppress of choke" , "Smother, suffocate" , "Asphyxiate - itself (anag)" .)