Having lost heart, fail test with colourful plants? (6)
I believe the answer is:
floral
'with colourful plants?' is the definition.
(with flowers)
'having lost heart fail test' is the wordplay.
'having lost heart' says to hollow out the word (remove centre letters).
'test' becomes 'oral' (oral is a type of examination).
'fail' with its centre removed is 'fl'.
'fl'+'oral'='FLORAL'
(Other definitions for floral that I've seen before include "Such tributes" , "Like daffodils" , "of a Roman goddess" , "bouquet" , "To do with flowers, perhaps an arrangement" .)