Small mountain, say (5)
I believe the answer is:
speak
'say' is the definition.
(thesaurus)
'small mountain' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mountain' becomes 'peak' (I've seen this before).
's'+'peak'='SPEAK'
(Other definitions for speak that I've seen before include "Utter words" , "give a lecture" , "Orate" , "Talk" , "Converse" .)