Decline irrationally to set us free (6)
I believe the answer is:
refuse
'decline irrationally' is the definition.
'refuse' can be an answer for 'decline' (synonyms). I am not certain of the 'irrationally' bit.
'set us free' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'us'+'free'='usfree'
'usfree' anagrammed gives 'REFUSE'.
'to' is the link.
Can you help me to learn more?
(Other definitions for refuse that I've seen before include "Decline to take the rubbish" , "Reject the waste material" , "Turn down - rubbish" , "Decline to accept - rubbish" , "deadwood" .)