Actually do something with endless knowledge (2,4)
I believe the answer is:
in fact
'actually' is the definition.
(synonyms)
'do something with endless knowledge' is the wordplay.
'do something' becomes 'act' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'endless' means to remove the last letter.
'knowledge' becomes 'info' (I've seen this before).
'info' with its final letter taken off is 'inf'.
'act' after 'inf' is 'IN FACT'.
(Other definitions for in fact that I've seen before include "Actually" , "'In reality (2,4)'" , "really!" .)