Device putting suitable ruler under notice (7)
I believe the answer is:
adapter
'device' is the definition.
(adapter is a kind of device)
'suitable ruler under notice' is the wordplay.
'suitable' becomes 'apt' (I've seen this before).
'ruler' becomes 'er' (abbreviation for Elizabeth Regina).
'under' is a charade indicator (letters next to each other).
'notice' becomes 'ad'.
'apt'+'er'='apter'
'apter' after 'ad' is 'ADAPTER'.
'putting' is the link.
(Other definitions for adapter that I've seen before include "'Apt, dear!' (anag.); a switching device" , "Dear Pat (anag)" , "sort of plug" , "I could modify" , "Plug attachment" .)