Hurry; he'll be round about one (3)
I believe the answer is:
hie
'hurry' is the definition.
('hie' can be a synonym of 'hurry')
'he'll be round about one' is the wordplay.
'he'll' becomes 'eh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be round' is a reversal indicator.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'eh' reversed gives 'he'.
'he' placed around 'i' is 'HIE'.
Can you help me to learn more?
(Other definitions for hie that I've seen before include "Go quickly (archaic)" , "Hurry" , "make haste" , "Be quick (lit.)" , "Make one's way quickly" .)