Plant, first thing for summer border (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'first thing for summer border' is the wordplay.
'first thing for' suggests taking the first letters.
'border' becomes 'edge' (synonyms).
The initial letter of 'summer' is 's'.
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Waterside plant" , "Marsh-dweller" , "Grass-like plant growing in wet places" , "Plant of wet ground" .)