Constrained old boy with cap guarding exit (9)
I believe the answer is:
obligated
'constrained' is the definition.
(similar in meaning)
'old boy with cap guarding exit' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (abbreviation) and 'o' is present in the answer.
'boy' could be 'ted' (short for Edward or Theodore) and 'ted' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for obligated that I've seen before include "in debt" , "Morally compelled" , "Bound" .)