Describe gusset for thermal clothing (3,5)
I believe the answer is:
set forth
'describe' is the definition.
(thesaurus)
'gusset for thermal clothing' is the wordplay.
'clothing' indicates the answer is hidden within the clue (the solution is covered or clothed by the clue).
'SET FORTH' is hidden within 'gusset for thermal'.
(Other definitions for set forth that I've seen before include "display" , "Depart" , "Begin journey or lay out" , "Proclaim" , "Embark" .)