I must keep small boy towing the line (6)
I believe the answer is:
onside
'towing the line' is the definition.
(towing the line means following an agreed message)
'i must keep small boy' is the wordplay.
'i' becomes 'one' (Roman numeral for one).
'must keep' indicates putting letters inside.
'small boy' becomes 'Sid' (short for Sidney; 'small' asks for an abbreviation).
'one' going around 'sid' is 'ONSIDE'.
(Other definitions for onside that I've seen before include "In agreement (colloq.)" , "legitimate" , "Legally positioned in game" , "(In football) not in forbidden zone" , "in a permitted position in sport" .)