Take orders from old boy you once sent up (4)
I believe the answer is:
obey
'take orders from' is the definition.
(I've seen this before)
'old boy you once sent up' is the wordplay.
'old boy' becomes 'ob'.
'you once' becomes 'ye' (I've seen this in other clues).
'sent up' is a reversal indicator.
'ye' written backwards gives 'ey'.
'ob'+'ey'='OBEY'
(Other definitions for obey that I've seen before include "Act in accordance with (law)" , "Follow the instructions" , "Do what one is told" , "Toe the line" , "Submit to authority" .)