On which to write letters about the chaplain (5)
I believe the answer is:
padre
'chaplain' is the definition.
(padre is a kind of chaplain)
'on which to write letters about' is the wordplay.
'on which to write letters' becomes 'pad' (letters can be written on a pad of paper).
'about' becomes 're' ('re' means regarding or about).
'pad'+'re'='PADRE'
'the' acts as a link.
(Other definitions for padre that I've seen before include "clerical worker" , "Military chaplain" , "Army serviceman?" , "Chaplain to armed forces" , "Priest, chaplain" .)