Old forward is beginning to employ special knowledge (9)
I believe the answer is:
expertise
'special knowledge' is the definition.
(I've seen this before)
'old forward is beginning to employ' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'forward' becomes 'pert' (I've seen this before).
'beginning to' says to take the initial letters.
The initial letter of 'employ' is 'e'.
'ex'+'pert'+'is'+'e'='EXPERTISE'
(Other definitions for expertise that I've seen before include "Craft" , "Adroitness" , "Mastery, proficiency" , "Technical skill" , "Pert sex, i.e. could become skilled knowledge" .)