Variable, premature annual (6)
I believe the answer is:
yearly
'annual' is the definition.
(synonyms)
'variable premature' is the wordplay.
'variable' becomes 'y' (algebra).
'premature' becomes 'early' (similar in meaning).
'y'+'early'='YEARLY'
(Other definitions for yearly that I've seen before include "in each revolution?" , "Every tenth of a decade" , "Perennial" , "At annual intervals" , "Every 12 months" .)