Heathen left beside fiend, I recollected (7)
I believe the answer is:
infidel
'heathen' is the definition.
(synonyms)
'left beside fiend i recollected' is the wordplay.
'left' becomes 'l' (common abbreviation).
'beside' says to put letters next to each other (I've seen this in other clues).
'recollected' indicates anagramming the letters.
'fiend'+'i'='fiendi'
'fiendi' is an anagram of 'infide'.
'l' after 'infide' is 'INFIDEL'.
(Other definitions for infidel that I've seen before include "One without religion (mainly archaic)" , "Pagan" , "Disbeliever" , "Literally, 9 [UNFAITHFUL]" , "Heathen" .)