Supporters in favour of addition to letter (5)
I believe the answer is:
props
'supporters' is the definition.
(things that prop up or support)
'favour of addition to letter' is the wordplay.
'favour of' becomes 'pr' (I can't explain this - if you can you should give a lot more credence to this answer).
'addition' becomes 'ps'.
'to' means one lot of letters go next to another (I've seen this in other clues).
'letter' becomes 'o'.
'ps' put after 'o' is 'ops'.
'pr'+'ops'='PROPS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for props that I've seen before include "Buttresses" , "Set" , "forwards" , "Forward types" , "Items used on stage" .)