Old bird beginning to twitch in ditch (4)
I believe the answer is:
moat
'ditch' is the definition.
(I know that moat is a type of trench)
'old bird beginning to twitch' is the wordplay.
'old bird' becomes 'moa' (extinct bird).
'beginning to' says to take the initial letters.
The initial letter of 'twitch' is 't'.
'moa'+'t'='MOAT'
'in' acts as a link.
(Other definitions for moat that I've seen before include "Water around castle" , "Ditch round castle" , "Defensive water-filled ditch" , ". . . part of defence" , "Trench around fortifications" .)