Aim to get fit, being wise (10)
I believe the answer is:
reasonable
'wise' is the definition.
(thesaurus)
'aim to get fit' is the wordplay.
'aim' becomes 'reason' (both can mean a motive).
'to get' says to put letters next to each other.
'fit' becomes 'able' (similar in meaning).
'reason'+'able'='REASONABLE'
'being' is the link.
(Other definitions for reasonable that I've seen before include "description of rationality" , "Fair - inexpensive" , "Fair, rational" , "Not excessive or extreme" , "Sound" .)