A footnote on English recess (4)
I believe the answer is:
apse
'recess' is the definition.
(apse is a kind of recess)
'a footnote on english' is the wordplay.
'footnote' becomes 'ps'.
'on' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'a'+'ps'+'e'='APSE'
(Other definitions for apse that I've seen before include "Church recess in lapsed condition" , "Strange peas in the church recess" , "Recess with domed rof in a church" , "part of cathedral" , "may be seen in church!" .)