Possibly run and see to make certain (6)
I believe the answer is:
ensure
'make certain' is the definition.
(synonyms)
'possibly run and see' is the wordplay.
'possibly' indicates an anagram.
'and' says to put letters next to each other.
'run'+'see'='runsee'
'runsee' is an anagram of 'ENSURE'.
'to' is the link.
(Other definitions for ensure that I've seen before include "Protect" , "See run (anag)" , "Make certain that (something occurs)" , "Make certain, clinch" , "Make certain (something will occur)" .)