Unit found in Somme trenches (5)
I believe the answer is:
metre
'unit' is the definition.
(metre is a kind of unit)
'in somme trenches' is the wordplay.
'in' says the answer is hidden in the clue.
'METRE' is hidden in the letters of 'somme trenches'.
'found' acts as a link.
(Other definitions for metre that I've seen before include "three and a bit of a fourth" , "determined by scanning" , "Approx 39 inches" , "100cm" , "1.094 yards" .)