They have pupils — for English, certainly (4)
I believe the answer is:
eyes
'they have pupils' is the definition.
(I've seen this before)
'english certainly' is the wordplay.
'english' becomes 'E' (abbreviation).
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'e'+'yes'='EYES'
'for' is the link.
(Other definitions for eyes that I've seen before include "Narrowly observes" , "Blemishes on potatoes" , "They have pupils" , "Parts of needles" , "Organs of vision" .)