Valley's width in metric unit (3)
I believe the answer is:
cwm
'valley's' is the definition.
(thesaurus)
'width in metric unit' is the wordplay.
'width' becomes 'w' (abbreviation in geometry).
'in' indicates putting letters inside.
'metric unit' becomes 'cm' (cm is a kind of metric unit).
'w' put within 'cm' is 'CWM'.
(Other definitions for cwm that I've seen before include "Valley (in Wales)" , "Bronwen's depression" , "Welsh mountainside hollow" , "Steep Welsh hollow" , "Welsh valley hollow" .)