Measure a lot of mountains (5)
I believe the answer is:
chain
'measure' is the definition.
(unit equal to 22 yards)
'a lot of mountains' is the wordplay.
'a lot' becomes 'c'.
'of mountains' becomes 'hain' (I am not sure about this - if you are sure you should believe this answer much more).
'c'+'hain'='CHAIN'
Can you help me to learn more?
(Other definitions for chain that I've seen before include "Series of connected links" , "A measure" , "Part of cycle" , "Group of atoms" , "Series of things linked together" .)