Flies are pulled up after donning immaculately pressed trousers, originally (7)
I believe the answer is:
diptera
'flies' is the definition.
(I've seen this before)
'are pulled up after donning immaculately pressed trousers originally' is the wordplay.
'pulled up' says the letters should be written backwards.
'after' is a charade indicator (letters next to each other).
'originally' says to take the initial letters.
The initial letters of 'donning immaculately pressed trousers' is 'dipt'.
'are' written backwards gives 'era'.
'era' put after 'dipt' is 'DIPTERA'.
(Other definitions for diptera that I've seen before include "Flighty little types" , "Flies" , "Insects" .)