Drily or in a dither, as a rule (10)
I believe the answer is:
ordinarily
'as a rule' is the definition.
(I've seen this before)
'drily or in a dither' is the wordplay.
'dither' is an anagram indicator (I've seen 'dithering' mean this).
'drily'+'or'+'in'+'a'='drilyorina'
'drilyorina' anagrammed gives 'ORDINARILY'.
(Other definitions for ordinarily that I've seen before include "following the custom" , "As a rule" , "In the usual run of things" .)