Measure part of mountain chain (4)
I believe the answer is:
inch
'measure' is the definition.
(inch is a kind of measure)
'part of mountain chain' is the wordplay.
'part of' indicates a hidden word.
'INCH' can be found hidden inside 'mountain chain'.
(Other definitions for inch that I've seen before include "Progress slowly" , "Short length" , "1/12 of foot" , "2.54 cm (approx.)" , "Non-metric measure" .)