Indicates informal speech is being written up (7)
I believe the answer is:
signals
'indicates' is the definition.
('signal' can be a synonym of 'indicate')
'informal speech is being written up' is the wordplay.
'informal speech' becomes 'slang' (I've seen this before).
'being written up' says the letters should be written backwards.
'slang'+'is'='slangis'
'slangis' written backwards gives 'SIGNALS'.
(Other definitions for signals that I've seen before include "(Traffic) lights" , "Flags, gestures" , "Actions that encode messages" , "Indications or gestures as information or warning" , "Coded communications" .)