Rigid measure? (5)
I believe the answer is:
ruler
'rigid measure?' is the definition.
(rigid tool for measuring things)
'rigid measure?' is the wordplay.
'rigid' becomes 'rule' (I can't explain this - if you can you should believe this answer much more).
'measure?' becomes 'R' (R is an example).
'rule'+'r'='RULER'
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Strip used in drawing straight lines" , "King, president etc." , "Chief" , "person holding sway?" , "One exercises dominion" .)