Soothe with unguent obtaining mercy occasionally (6)
I believe the answer is:
becalm
'soothe' is the definition.
(thesaurus)
'unguent obtaining mercy occasionally' is the wordplay.
'unguent' becomes 'balm' (synonyms).
'obtaining' is an insertion indicator.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternate letters of 'mercy' are 'ec'.
'balm' going around 'ec' is 'BECALM'.
'with' is the link.
(Other definitions for becalm that I've seen before include "Leave (sailing ship) without wind" , "don't get the wind up" , "Leave (sailing ship) unable to move" , "Render motionless from lack of wind" , "Leave (sailing ship) deprived of power" .)