Really move stern of boat into berth (5)
I believe the answer is:
motor
'really move' is the definition.
'motor' can be an answer for 'move' (motoring is a kind of moving). I am unsure of the 'really' bit.
'stern of boat into berth' is the wordplay.
'stern of' says to take the final letters (stern can mean back).
'into' means one lot of letters goes inside another.
'berth' becomes 'moor' (synonyms).
The final letter of 'boat' is 't'.
't' put within 'moor' is 'MOTOR'.
Can you help me to learn more?
(Other definitions for motor that I've seen before include "Make good progress" , "Drive" , "Engine - relating to movement" , "(Transport using) engine" , "go quickly" .)