Revolutionary sects primarily infiltrated an island nation (2,5)
I believe the answer is:
st lucia
'island nation' is the definition.
(St Lucia is an island)
'revolutionary sects primarily infiltrated an' is the wordplay.
'revolutionary' says the letters should be written backwards (letters revolve around).
'sects' becomes 'cults' (cult is a type of religious sect).
'primarily' says to take the initial letters.
The first letters of 'infiltrated an' is 'ia'.
'cults' written backwards gives 'stluc'.
'stluc'+'ia'='ST LUCIA'
(Other definitions for st lucia that I've seen before include "Windward Isles country to the south of Martinique" , "'Volcanic island in the Windward Isles (2,5)'" , "Caribbean country" , "Commonwealth country in the Windward Islands" .)