Son horsing about with grandpa makes bloomer (10)
I believe the answer is:
snapdragon
'bloomer' is the definition.
(snapdragon is a kind of flowering plant)
'son horsing about with grandpa makes' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'horsing about' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'makes' is an anagram indicator.
'grandpa' is an anagram of 'napdrag'.
'on' after 'napdrag' is 'napdragon'.
's'+'napdragon'='SNAPDRAGON'
Can you help me to learn more?
(Other definitions for snapdragon that I've seen before include "One blooming" , "one's found in the garden" , "that's in the garden?" , "Perhaps Christmas entertainment" , "one found in bed" .)