Writer is one infiltrating Triads in disguise (7)
I believe the answer is:
diarist
'writer' is the definition.
(diarist is a kind of writer)
'one infiltrating triads in disguise' is the wordplay.
'one' becomes 'i' (Roman numeral).
'infiltrating' indicates putting letters inside.
'in disguise' indicates anagramming the letters.
'triads' is an anagram of 'diarst'.
'i' put within 'diarst' is 'DIARIST'.
'is' is the link.
(Other definitions for diarist that I've seen before include "One as Evelyn, Kilvert" , "Pepys, for example" , "Someone like Pepys" , "Writer" , "eg Samuel Pepys" .)