Soothe relative with a kiss (5)
I believe the answer is:
relax
'soothe' is the definition.
(soothe can mean to relax)
'relative with a kiss' is the wordplay.
'relative' becomes 'rel' (mathematical abbreviation).
'with' says to put letters next to each other.
'kiss' becomes 'x' (X can represent a kiss).
'rel'+'a'+'x'='RELAX'
(Other definitions for relax that I've seen before include "Take it easy, don't be so tense" , "Unbend" , "Abate" , "Loosen up, slow down" , "Rest" .)