Swear on screen (5)
I believe the answer is:
blind
This clue is a double definition.
'swear' is the first definition.
(to 'eff and blind' is to swear)
'screen' is the second definition.
(synonyms)
'on' acts as a link.
(Other definitions for blind that I've seen before include "relying on instruments" , "deception" , "how very drunk one is" , "shade" , "Unable to see - screen" .)