Jack, John and Miles covered in flowers (6)
I believe the answer is:
abloom
'covered in flowers' is the definition.
'jack john and miles' is the wordplay.
'jack' becomes 'ab'.
'john' becomes 'loo' (loo is a kind of john).
'and' says to put letters next to each other.
'miles' becomes 'm' (common abbreviation).
'ab'+'loo'+'m'='ABLOOM'
(Other definitions for abloom that I've seen before include "Flowering" , "Healthy" , "Flourishing" , "In flower" , "covered in flowers?" .)