Border teams helped to build new town (5,9)
I believe the answer is:
hemel hempstead
'new town' is the definition.
(I've seen this before)
'border teams helped to build' is the wordplay.
'border' becomes 'hem' (I've seen this before).
'to build' indicates an anagram.
'teams'+'helped'='teamshelped'
'teamshelped' with letters rearranged gives 'elhempstead'.
'hem'+'elhempstead'='HEMEL HEMPSTEAD'
(Other definitions for hemel hempstead that I've seen before include "Hertfordshire town" , "new town" .)