Navy going after a company as a source of oak (5)
I believe the answer is:
acorn
'a source of oak' is the definition.
(oaks grow from acorns)
'navy going after a company' is the wordplay.
'navy' becomes 'RN' (Royal Navy).
'going after' says to put letters next to each other.
'company' becomes 'co' (abbreviation).
'a'+'co'='aco'
'rn' put after 'aco' is 'ACORN'.
'as' acts as a link.
(Other definitions for acorn that I've seen before include "Model in stone" , "From which a tall tree may grow" , "Dirt on a liquid surface" , "Source of a great oak" , "Oak seed" .)