Letter from a church covering appeal (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter)
'a church covering appeal' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'covering' means one lot of letters goes inside another.
'appeal' becomes 'it'.
'a'+'ch'='ach'
'ach' enclosing 'it' is 'AITCH'.
'from' is the link.
(Other definitions for aitch that I've seen before include "Aspirated letter" , "Letter sound that's often dropped?" , "bone" , "the start of hives?" , "first bit of hymn?" .)