Free from duty: on leave we hear (7)
I believe the answer is:
relieve
'free from duty' is the definition.
(relieving is a kind of freeing from duty)
'on leave we hear' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
The remaining letters 'lieve' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for relieve that I've seen before include "Lighten a burden or raise a seige" , "Make (pain) less severe" , "Alleviate, assuage" , "Alleviate pain or stress" , "Make less tedious, bring aid to (besieged place)" .)