Race round, at liberty to be happy (8)
I believe the answer is:
carefree
'happy' is the definition.
(synonyms)
'race round at liberty' is the wordplay.
'round' indicates anagramming the letters.
'at liberty' becomes 'free' (similar in meaning).
'race' is an anagram of 'care'.
'care'+'free'='CAREFREE'
'to be' acts as a link.
(Other definitions for carefree that I've seen before include "not concerned?" , "Without responsibility or anxiety" , "Not worried" , "Light-hearted, no worries" , "Without a worry in the world" .)