Pay nothing to fill ditch — free! (9)
I believe the answer is:
footloose
'free' is the definition.
(I've seen this before)
'pay nothing to fill ditch' is the wordplay.
'pay' becomes 'foot' (as in footing the bill).
'nothing' becomes 'O' (O resembles 0).
'to fill' indicates putting letters inside.
'ditch' becomes 'lose' (both can mean to abandon).
'o' inserted into 'lose' is 'loose'.
'foot'+'loose'='FOOTLOOSE'
(Other definitions for footloose that I've seen before include "Able to travel freely and do as one pleases" , "Eager to travel" , "Able to go" , "Free from ties" , "without responsibility?" .)