Tried, as a son should (6)
I believe the answer is:
sought
'tried' is the definition.
('seek' can be a synonym of 'try')
'a son should' is the wordplay.
'a son' becomes 's'.
'should' becomes 'ought' (synonyms).
's'+'ought'='SOUGHT'
'as' acts as a link.
(Other definitions for sought that I've seen before include "Tried to find" , "Looked for" , "Wanted" , "Attempted to find" , "Searched for - investigated - desired" .)