The head of staff frequently will relent (6)
I believe the answer is:
soften
'relent' is the definition.
(I know that relent can be written as soften)
'the head of staff frequently' is the wordplay.
'the head of' indicates taking the first letters.
'frequently' becomes 'often' (synonyms).
The initial letter of 'staff' is 's'.
's'+'often'='SOFTEN'
'will' is the link.
(Other definitions for soften that I've seen before include "Temper" , "Become gentler" , "become less rough" , "Tenderise" , "Make more plastic" .)