Fake illness a gremlin concocted (8)
I believe the answer is:
malinger
'fake illness' is the definition.
'a gremlin concocted' is the wordplay.
'concocted' indicates an anagram.
'a'+'gremlin'='agremlin'
'agremlin' anagrammed gives 'MALINGER'.
(Other definitions for malinger that I've seen before include "Pretend to be sick" , "Slack in germinal fashion" , "Germinal (anag.)" , "Avoid duty by pretending sickness" , "skive off school, maybe" .)