Priceless mineral found in mountain pass (7)
I believe the answer is:
comical
'priceless' is the definition.
(thesaurus)
'mineral found in mountain pass' is the wordplay.
'mineral' becomes 'mica' (mica is a kind of mineral).
'found in' means one lot of letters goes inside another.
'mountain pass' becomes 'col' (col is a kind of mountain pass).
'mica' going into 'col' is 'COMICAL'.
(Other definitions for comical that I've seen before include "priceless" , "Witty" , "Humorous" , "'Amusing, funny (7)'" , "Amusing or droll" .)