To economise, dig ditch again (8)
I believe the answer is:
retrench
'ditch again' is the definition.
('trench' can be a synonym for 'ditch' so perhaps 'retrench' is OK for 'ditch again')
'to economise dig' is the wordplay.
'to economise' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'dig' becomes 'trench' (trenching is a kind of digging).
're'+'trench'='RETRENCH'
Can you help me to learn more?
(Other definitions for retrench that I've seen before include "Once more dig deep" , "Reduce one's workforce" , "Cut down expenditure" , "Cut back (on costs)" , "Cut back spending" .)