Calm a couple of sons united by length of life (7)
I believe the answer is:
assuage
'calm' is the definition.
(assuaging is a kind of calming)
'a couple of sons united by length of life' is the wordplay.
'couple of' says to double the letters.
'sons' becomes 's'.
'united' becomes 'u'.
'by' says to put letters next to each other.
'length of life' becomes 'age' (I've seen this before).
's' duplicated is 'ss'.
'a'+'ss'+'u'+'age'='ASSUAGE'
(Other definitions for assuage that I've seen before include "Make less intense; satisfy" , "Make less severe, say thirst" , "'Alleviate, appease (7)'" , "Soothe, make less severe" , "Ease or relieve, say thirst" .)