Sibling allowed to go on excursion (7)
I believe the answer is:
triplet
'sibling' is the definition.
(I know that triplet is a type of sibling)
'allowed to go on excursion' is the wordplay.
I cannot really understand how this works, but
'allowed' could be 'let' (to let is to allow or permit) and 'let' is present in the answer.
'excursion' could be 'trip' (a trip is an excursion or outing) and 'trip' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for triplet that I've seen before include "One of a group of three" , "One of a small group" , "Member of large family" , "Not the only baby in the group" , "Handful of notes" .)