Female on isle stops devils (6)
I believe the answer is:
fiends
'devils' is the definition.
(fiend can mean a demon)
'female on isle stops' is the wordplay.
'female' becomes 'f' (common abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'isle' becomes 'i' (geographical abbreviation).
'stops' becomes 'ends' ('end' can be a synonym of 'stop').
'f'+'i'+'ends'='FIENDS'
(Other definitions for fiends that I've seen before include "Very wicked or cruel people" , "Evil spirits or wicked people" , "Devils or cruel people" , "Addicts; devotees" , "Cruel people or evil spirits" .)