Being fit is a bit of a blessing (4)
I believe the answer is:
able
'being fit' is the definition.
('able' can be similar in meaning to 'fit')
'a bit of a blessing' is the wordplay.
'bit of a blessing' becomes 'ble' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'ble'='ABLE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for able that I've seen before include "Fully competent" , "Capable or proficient" , "See 10" , "Proficient; skilful" , "Of high proficiency" .)