Get sick? Had to return flower? (6)
I believe the answer is:
dahlia
'flower?' is the definition.
(dahlia is a kind of flower)
'get sick? had to return' is the wordplay.
'get sick?' becomes 'ail' (I can't explain this - if you can you should give a lot more credence to this answer).
'to return' says the letters should be written in reverse.
'ail'+'had'='ailhad'
'ailhad' backwards is 'DAHLIA'.
Can you help me to learn more?
(Other definitions for dahlia that I've seen before include "Flower, often ball-shaped" , "Strangely, Ali had this flower" , "Tuberous plant with showy flowers" , "Lia had this flower, strangely" , "Garden plant with large brightly-coloured flowers" .)