Desire new shirt and trousers, primarily (6)
I believe the answer is:
thirst
'desire' is the definition.
(I know that thirst is a type of desire)
'new shirt and trousers primarily' is the wordplay.
'new' indicates an anagram.
'and' is a charade indicator (letters next to each other).
'primarily' suggests taking the first letters.
The first letter of 'trousers' is 't'.
'shirt' is an anagram of 'thirs'.
'thirs'+'t'='THIRST'
(Other definitions for thirst that I've seen before include "Quaff to slake it" , "T-shirt (anag)" , "Physiological need to drink" , "Craving for liquid" , "Long" .)