Improvise with a seaman holding 551 (2,3)
I believe the answer is:
ad lib
'improvise' is the definition.
(I know that improvise can be written as ad-lib)
'seaman holding 551' is the wordplay.
'seaman' becomes 'ab' (short for able seaman).
'holding' means one lot of letters goes inside another.
'551' becomes 'dli' (I've seen this in another clue).
'ab' going around 'dli' is 'AD-LIB'.
'with a' is the link.
(Other definitions for ad lib that I've seen before include "As You Like It?" , "Improvised" , "Addition to speech" , "busk" , "Make an impromptu speech" .)