Volunteer thought cardinal collars irrational (5)
I believe the answer is:
opine
'volunteer thought cardinal collars irrational' is the definition.
I can't judge whether this defines the answer.
'cardinal collars irrational' is the wordplay.
'cardinal' becomes 'one' (a cardinal number).
'collars' means one lot of letters goes inside another (to collar can mean to apprehend or capture).
'irrational' becomes 'pi' (an irrational number).
'one' placed around 'pi' is 'OPINE'.
Can you help me to learn more?
(Other definitions for opine that I've seen before include "Give opinion" , "Give one's view" , "Express a point of view" , "Express one's opinion openly" , "Consider" .)