Mean to swear before the years (7)
I believe the answer is:
average
'mean' is the definition.
(synonyms)
'swear before the years' is the wordplay.
'swear' becomes 'aver' (synonyms).
'before' means one lot of letters go next to another.
'the years' becomes 'age' (synonyms).
'aver'+'age'='AVERAGE'
'to' is the link.
(Other definitions for average that I've seen before include "About normal or usual" , "Moderate" , "Mean - medium" , "Run of the mill" , "It's common" .)