Ages working out sum for philosopher (7)
I believe the answer is:
erasmus
'philosopher' is the definition.
(I've seen this before)
'ages working out sum' is the wordplay.
'ages' becomes 'eras' (I've seen this in another clue).
'working out' indicates an anagram (an anagram must be worked out from the letters).
'sum' is an anagram of 'mus'.
'eras'+'mus'='ERASMUS'
'for' is the link.
(Other definitions for erasmus that I've seen before include "Learned person" , "religious scholar" , "Old theologian" , "Humanist critical of Church" , "Dutch philosopher" .)