Realises has too sedentary an occupation (11)
I believe the answer is:
'realises' is the definition.
('understand' can be a synonym of 'realize')
'too sedentary an occupation' is the wordplay.
I cannot really see how this works, but
'too' could be 'and' (I've seen this in another clue) and 'and' is present in the answer.
This explanation may well be incorrect...
'has' acts as a link.
Can you help me to learn more?
(Another definition for understands that I've seen is " Appreciates".)