Hard work keeping a number in tow (7)
I believe the answer is:
travail
'hard work' is the definition.
('travail' can be a synonym of 'hard work')
'keeping a number in tow' is the wordplay.
'keeping' is an insertion indicator.
'number' becomes 'v' (V is an example**).
'in' is an insertion indicator.
'tow' becomes 'trail' (trailing is a kind of towing).
'v' placed into 'trail' is 'travil'.
'a' placed inside 'travil' is 'TRAVAIL'.
(Other definitions for travail that I've seen before include "In old use, the pains of childbirth" , "Labour, toil" , "Exertion, elbow grease" , "Old word for pangs of childbirth" , "Painful labour" .)