Train? There’s one in a minute (3)
I believe the answer is:
'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).
'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" .)