Crumb, thing under lid of pan (8)
I believe the answer is:
particle
'crumb' is the definition.
(thesaurus)
'thing under lid of pan' is the wordplay.
'thing' becomes 'article' (I've seen this before).
'under' is a charade indicator (letters next to each other).
'lid of' says to take the initial letters (in a down clue, the lid could be the top letter).
The first letter of 'pan' is 'p'.
'article' after 'p' is 'PARTICLE'.
(Other definitions for particle that I've seen before include "Eg, a proton" , "Molecule" , "Small piece of matter" , "Prelatic (anag.)" , "tiny portion" .)