Drop tin first that's glowing (10)
I believe the answer is:
candescent
'glowing' is the definition.
(I've seen this in another clue)
'drop tin first' is the wordplay.
'drop' becomes 'descent' (I've seen this in another clue).
'tin' becomes 'can' (synonyms).
'first' says to put letters next to each other.
'descent' after 'can' is 'CANDESCENT'.
'that's' acts as a link.
(Other definitions for candescent that I've seen before include "It's white-hot" , "Glowing" .)