Sudden pressure in routine, following sailor (6)
I believe the answer is:
abrupt
'sudden' is the definition.
('abrupt' can be similar in meaning to 'sudden')
'pressure in routine following sailor' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'in' means one lot of letters goes inside another.
'routine' becomes 'rut' (rut is a kind of routine**).
'following' says to put letters next to each other.
'sailor' becomes 'ab' (abbreviation for able seaman).
'p' going into 'rut' is 'rupt'.
'rupt' put after 'ab' is 'ABRUPT'.
(Other definitions for abrupt that I've seen before include "Short, sharp or sudden" , "using minimal language?" , "Unexpected -- precipitous" , "Terse, curt" , "Sudden, maybe discourteous" .)