Had problem - large one - a flower (6)
I believe the answer is:
dahlia
'flower' is the definition.
(I know that dahlia is a type of flower)
'had problem large one a' is the wordplay.
'problem' indicates an anagram.
'large' becomes 'L' (eg in clothes sizes).
'one' becomes 'i' (Roman numeral).
'had' is an anagram of 'dah'.
'dah'+'l'+'i'+'a'='DAHLIA'
(Other definitions for dahlia that I've seen before include "In bed, root" , "Had Lia this peculiar flower?" , "Lia had this flower, strangely" , "Strangely, Ali had this flower" , "Peculiar Ali had this flower" .)