Initial word a friend mentioned firstly (7)
I believe the answer is:
acronym
'initial word' is the definition.
(made with initials)
'a friend mentioned firstly' is the wordplay.
'friend' becomes 'crony' (crony is a kind of friend).
'firstly' indicates taking the first letters.
The initial letter of 'mentioned' is 'm'.
'a'+'crony'+'m'='ACRONYM'
(Other definitions for acronym that I've seen before include "My acorn may generate a new word from first letters" , "UNESCO, say" , "Eg, Nato, Sars" , "Radar or Nato, maybe" , "Word made up of initials" .)