Extremely foul tramp’s lost outside (5)
I believe the answer is:
ultra
'extremely' is the definition.
'foul tramp's lost outside' is the wordplay.
'outside' indicates the answer is hidden within the clue (a word is around or outside the answer).
'ULTRA' can be found hidden inside 'foul tramps lost'.
(Other definitions for ultra that I've seen before include "Extremely; an extremist" , "Prefix meaning extreme or excessive" , "Extreme (as in UV)" , "Saharan dweller" , "Far beyond the norm" .)