Grandpa's son arranged flowers (11)
I believe the answer is:
snapdragons
'flowers' is the definition.
(snapdragon is a kind of flower)
'grandpa's son arranged' is the wordplay.
'arranged' is an anagram indicator (letters rearranged).
'grandpas'+'son'='grandpasson'
'grandpasson' is an anagram of 'SNAPDRAGONS'.
(Other definitions for snapdragons that I've seen before include "Flowers" , "bloomers" .)