... that's sensible, when teenager is confined to base! (8)
I believe the answer is:
grounded
'that's sensible' is the definition.
I can't tell whether this definition defines the answer.
'teenager is confined to base' is the wordplay.
'teenager' becomes 'de' (I can't explain this - if you can you should give a lot more credence to this answer).
'is confined to' means one lot of letters goes inside another.
'base' becomes 'ground' (synonyms).
'de' put inside 'ground' is 'GROUNDED'.
'when' is the link.
Can you help me to learn more?
(Other definitions for grounded that I've seen before include "Earthed" , "Based; forbidden to fly" , "prevented from leaving home" , "Not allowed to fly" , "Punished teenager by confining to house - poor underdog" .)