Casual greeting from old sailors in China (7)
I believe the answer is:
morning
'casual greeting' is the definition.
'old sailors in china' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'in' means one lot of letters goes inside another.
'china' becomes 'ming' (I've seen this before).
'o'+'rn'='orn'
'orn' placed within 'ming' is 'MORNING'.
'from' acts as a link.
(Other definitions for morning that I've seen before include "Period up to noon" , "Time between dawn and noon" , "First part of day" , "early part of day" , "Forenoon" .)