Learn about officer with hurry to get fitter (9)
I believe the answer is:
healthier
'fitter' is the definition.
('healthy' can be a synonym of 'fit')
'learn about officer with hurry' is the wordplay.
'learn' becomes 'hear' (synonyms).
'about' means one lot of letters goes inside another.
'officer' becomes 'lt' (abbreviation for lieutenant).
'with' means one lot of letters go next to another.
'hurry' becomes 'hie' (historical word meaning go quickly or hasten).
'lt'+'hie'='lthie'
'hear' placed around 'lthie' is 'HEALTHIER'.
'to get' acts as a link.
(Other definitions for healthier that I've seen before include "Fitter" , "less prone to disease" , "More in the pink" .)