Room to manoeuvre a small line over stern of ferry
I believe the answer is:
leeway
'room to manoeuvre' is the definition.
(I've seen this before)
'a small line over stern of ferry' is the wordplay.
'small' becomes 'wee' (Scottish term for small).
'line' becomes 'l' (used when specifying particular lines from a poem).
'over' says the letters should be written in reverse.
'stern of' suggests the final letters (stern can mean back).
The final letter of 'ferry' is 'y'.
'a'+'wee'+'l'='aweel'
'aweel' backwards is 'leewa'.
'leewa'+'y'='LEEWAY'
(Other definitions for leeway that I've seen before include "Permissible difference, allowing some freedom to move" , "Allowance" , "Room to manoeuvre, as for a ship" , "Give" , "Limited room for manoeuvre" .)