Mischief-makers infiltrated by the Spanish forces (6)
I believe the answer is:
impels
'forces' is the definition.
('impel' can be a synonym of 'force')
'mischief-makers infiltrated by the spanish' is the wordplay.
'mischief makers' becomes 'imps' (I've seen this in another clue).
'infiltrated by' indicates putting letters inside.
'the spanish' becomes 'el' ('the' in Spanish).
'imps' going around 'el' is 'IMPELS'.
(Other definitions for impels that I've seen before include "Urges forward" , "Forces, urges to action" , "Urges or drives to do something" , "Drives on in simple way" , "Spurs" .)