The British oddly unable to follow routine (5)
I believe the answer is:
usual
'routine' is the definition.
(synonyms)
'the british oddly unable to follow' is the wordplay.
'the british' becomes 'us' (the crossword is in a British publication).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'to follow' is a charade indicator (letters next to each other).
The alternating letters of 'unable' are 'ual'.
'us'+'ual'='USUAL'
(Other definitions for usual that I've seen before include "Expected" , "In accordance with regular practice" , "Customary, routine" , "familiar" , "Drink repeatedly taken" .)