To us, an early start shows purpose (3)
I believe the answer is:
use
'purpose' is the definition.
(synonyms)
'to us an early start' is the wordplay.
'start' indicates taking the first letters.
The initial letter of 'early' is 'e'.
'us'+'e'='USE'
'shows' is the link.
(Other definitions for use that I've seen before include "Apply, employ" , "Consume; deploy" , "Function, purpose" , "Sue for employment" , "Take drugs regularly" .)