Brown bug finally turned into small fly (4)
I believe the answer is:
gnat
'small fly' is the definition.
(I've seen this before)
'brown bug finally turned' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown**).
'finally' says to take the final letters.
'turned' says the letters should be written backwards.
The final letter of 'bug' is 'g'.
'tan'+'g'='tang'
'tang' written backwards gives 'GNAT'.
'into' acts as a link.
(Other definitions for gnat that I've seen before include "Biting pest" , "little irritation" , "Small (swarming) fly" , "Any of various small biting flies" , "Tang of a biting fly" .)