Wise men proliferate in Civil Service: they're picked to change minds? (5,9)
I believe the answer is:
magic mushrooms
'picked to change minds?' is the definition.
I know nothing about this answer so I can't judge whether this works.
'wise men proliferate in civil service' is the wordplay.
'wise men' becomes 'magi' (the three wise men from the nativity).
'proliferate' becomes 'mushroom' (I've seen this before).
'in' is an insertion indicator.
'civil service' becomes 'CS' (abbreviation).
'mushroom' inserted into 'cs' is 'cmushrooms'.
'magi'+'cmushrooms'='MAGIC MUSHROOMS'
'they're' acts as a link.
Can you help me to learn more?
(Another definition for magic mushrooms that I've seen is " you could trip on these".)