Strip, changing under ship (7)
I believe the answer is:
undress
'strip' is the definition.
(I know that strip can be written as undress)
'changing under ship' is the wordplay.
'changing' indicates an anagram.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'under' is an anagram of 'undre'.
'undre'+'ss'='UNDRESS'
(Other definitions for undress that I've seen before include "Take off clothing, strip" , "Strip, take off clothes" , "Remove clothes, strip" , "Take one's clothes off" , "Remove one's clothes" .)