It can be grown by a veteran for a companion (5)
I believe the answer is:
vetch
'it can be grown' is the definition.
The definition suggests a singular noun which matches the answer.
'a veteran for a companion' is the wordplay.
'a veteran' becomes 'vet' (abbreviation).
'for' says to put letters next to each other.
'a companion' becomes 'ch' (Companion of Honour).
'vet'+'ch'='VETCH'
'by' is the link.
Can you help me to learn more?
(Other definitions for vetch that I've seen before include "Wild climbing plant" , "Purple climbing plant" , "Climbing plant of the pea family, cultivated as fodder" , "Climber" , "Weed" .)