Midshipman marshalled tide so gently! (4,4,2)
I believe the answer is:
easy does it
'gently' is the definition.
(I've seen this before)
'midshipman marshalled tide so' is the wordplay.
'midshipman' becomes 'easy' (ref to Frederick Marryat novel Mr Midshipman Easy).
'marshalled' indicates anagramming the letters (I've seen 'marshals' mean this (marshal can mean to gather)).
'tide'+'so'='tideso'
'tideso' with letters rearranged gives 'doesit'.
'easy'+'doesit'='EASY DOES IT'
(Other definitions for easy does it that I've seen before include "Take your time" , "relax" , "What satisfies "Simple (4)"?" , "Be careful" , "Gently" .)