Seamen following mobile unit, one after the other (2,4)
I believe the answer is:
in turn
'one after the other' is the definition.
(I've seen this before)
'seamen following mobile unit' is the wordplay.
'seamen' becomes 'rn' (I can't explain this - if you can you should give a lot more credence to this answer).
'following' is a charade indicator (letters next to each other).
'mobile' indicates anagramming the letters (mobile can mean moving).
'unit' with letters rearranged gives 'intu'.
'rn' after 'intu' is 'IN TURN'.
Can you help me to learn more?
(Other definitions for in turn that I've seen before include "in an orderly fashion" , "orderly treatment" , "One after another" , "not out of place" , "a succession of things happen" .)