Some grime found inside, it's a headache (6)
I believe the answer is:
megrim
'a headache' is the definition.
(megrim is a kind of headache)
'some grime found inside' is the wordplay.
'inside' says the answer is hidden in the clue.
'MEGRIM' is hidden in the letters of 'some grime found'.
'it's' acts as a link.
(Other definitions for megrim that I've seen before include "Fish" , "Low spirits and a flatfish" , "Deep-water flatfish, I'm germ (anag.)" , "Type of turbot" , "Severe headache" .)