Amateur, through experiment, almost flies (7)

'flies' is the definition.
('aviate' can be a synonym of 'fly')

'amateur through experiment almost' is the wordplay.
'amateur' becomes 'a'.
'through' becomes 'via' (I've seen this before**).
'experiment' becomes 'test' (test is a kind of experiment).
'almost' means to remove the last letter (most of the word but not all of it).
'test' with its final letter removed is 'tes'.
'a'+'via'+'tes'='AVIATES'