Act on small documents (5)
I believe the answer is:
deeds
'documents' is the definition.
(deed is a kind of document)
'act on small' is the wordplay.
'act' becomes 'deed' (deed is a kind of act).
'on' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'deed'+'s'='DEEDS'
(Other definitions for deeds that I've seen before include "Activities" , "Actions" , "Acts" , "Exploits" , "Feats" .)