Speaking plainly, not top level (4)
I believe the answer is:
rank
'level' is the definition.
(I've seen this before)
'speaking plainly not top' is the wordplay.
'speaking plainly' becomes 'frank'.
'not top' means to remove the first letter.
'frank' with its initial letter taken away is 'RANK'.
(Other definitions for rank that I've seen before include "The military row is coarsely overgrown" , "(Taxi) stand" , "film executive" , "Stinking" , "Social class" .)