Soldier perhaps infiltrated by good English spy (5)
I believe the answer is:
agent
'spy' is the definition.
(thesaurus)
'soldier perhaps infiltrated by good english' is the wordplay.
'soldier perhaps' becomes 'ant' (soldier is a type of ant).
'infiltrated by' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'english' becomes 'e' (abbreviation).
'g'+'e'='ge'
'ant' going around 'ge' is 'AGENT'.
(Other definitions for agent that I've seen before include "Instrument" , "Delegate" , "Business manager" , "Double-zero-seven, for example" , "Author" .)