A son certain to get guarantee (6)
I believe the answer is:
assure
'guarantee' is the definition.
(I know that guarantee is a more specific form of assure)
'a son certain to get' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'certain' becomes 'sure' (synonyms).
'to get' indicates putting letters inside.
's' going into 'sure' is 'ssure'.
'a'+'ssure'='ASSURE'
(Other definitions for assure that I've seen before include "Make (something) certain to happen" , "Impart confidence" , "Give confidence to" , "Convince, promise" , "Say positively" .)