Theme of chapter probing sea and air transport (7)
I believe the answer is:
subject
'theme' is the definition.
(I know that theme can be written as subject)
'chapter probing sea and air transport' is the wordplay.
I cannot quite see how this works, but
'chapter' could be 'c' (abbreviation) and 'c' is found within the answer.
'transport' could be 'sub' (sub is a kind of transport) and 'sub' is present in the answer.
The remaining letters 'jet' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for subject that I've seen before include "person photographed" , "Person under 26?" , "Expose; field of study" , "guinea pig" , "Citizenship' at school maybe" .)