Harden, if held captive by authoritarian, blowing top (6)
I believe the answer is:
ossify
'harden' is the definition.
(I've seen this in another clue)
'if held captive by authoritarian blowing top' is the wordplay.
'held captive by' indicates putting letters inside (inserted letters are kept captive).
'authoritarian' becomes 'bossy' (extremely controlling).
'blowing top' means to remove the first letter.
'bossy' with its first letter taken away is 'ossy'.
'if' going within 'ossy' is 'OSSIFY'.
(Other definitions for ossify that I've seen before include "Get hard" , "Change into bone, or become unprogressive" , "Turn to bone with age" , "become inflexible" , "stop development" .)