Philosopher picking up on what? (5)
I believe the answer is:
hegel
'philosopher' is the definition.
(German philosopher Georg Wilhelm Friedrich Hegel)
'picking up on what?' is the wordplay.
'picking up' says the letters should be written in reverse.
'on' becomes 'leg' (cricket term, fielding positions).
'what?' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'leg'+'eh'='legeh'
'legeh' backwards is 'HEGEL'.
(Other definitions for hegel that I've seen before include "Georg Wilhelm ---, Ger. philosopher" , "German philosopher, d. 1831" , "German 'dialectic' philosopher" .)