Not a mirage perhaps but ingrained dirt (5)
I believe the answer is:
grime
'ingrained dirt' is the definition.
(I've seen this before)
'not a mirage perhaps' is the wordplay.
'not' indicates named letters should be taken away.
'perhaps' is an anagram indicator.
'mirage' with 'a' removed is 'mirge'.
'mirge' with letters rearranged gives 'GRIME'.
'but' acts as a link.
(Other definitions for grime that I've seen before include "Filth, dirt" , "an ingrained phenomenon" , "Dirt, grease" , "Ingrained dirt" , "Muck" .)