Interpret old poet (7)
I believe the answer is:
expound
'interpret' is the definition.
(I've seen this before)
'old poet' is the wordplay.
'old' becomes 'ex'.
'poet' becomes 'pound' (I've seen this before).
'ex'+'pound'='EXPOUND'
(Other definitions for expound that I've seen before include "Present and explain (a theory)" , "Set out in a detailed and learned way" , "Put forward detailed argument" , "Elucidate" , "Spell out" .)