Train? There’s one in a minute (3)
I believe the answer is:
aim
'train?' is the definition.
(I've seen this before)
'one in a minute' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'minute' becomes 'm' (abbreviation).
'a'+'m'='am'
'i' put inside 'am' is 'AIM'.
'there's' is the link.
(Other definitions for aim that I've seen before include "Set sights on, say target" , "Object; intend" , "intent" , "Steady, ... fire" , "Set sights on, target or goal" .)