First summer has no casual wear (6)
I believe the answer is:
slacks
'casual wear' is the definition.
(I've seen this before)
'first summer has no' is the wordplay.
'first summer' becomes 's' (1st letter of 'summer').
'has no' becomes 'lacks'.
's'+'lacks'='SLACKS'
(Other definitions for slacks that I've seen before include "takes it easy" , "Things to wear" , "Pants for casual wear" , "Avoids work - trousers" , "Loose casual trousers" .)