Letter showing way inside English mansion (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that epistle is a type of letter)
'way inside english mansion' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'inside' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'mansion' becomes 'pile' (I've seen this in other clues).
'e'+'pile'='epile'
'st' put within 'epile' is 'EPISTLE'.
'showing' acts as a link.
(Other definitions for epistle that I've seen before include "Writing" , "Long formal letter (found in the New Testament?)" , "may read this" , "Apostle's letter" , "may be written off" .)