Excursion allowed for one child in three? (7)
I believe the answer is:
triplet
'three?' is the definition.
('triplet' can be a synonym of 'three')
'excursion allowed' is the wordplay.
'excursion' becomes 'trip' (I've seen this before).
'allowed' becomes 'let' (to let is to allow or permit).
'trip'+'let'='TRIPLET'
'for one child in' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for triplet that I've seen before include "One of three born at the same time" , "One of three children" , "One of several arriving together" , "one arriving with two others" , "One of three born together" .)