Something in chest I’m thinking is on back of ship (7)
I believe the answer is:
sternum
'something in chest' is the definition.
'i'm thinking is on back of ship' is the wordplay.
'i'm thinking' becomes 'er' (expression used when hesitating).
'is on' means one lot of letters goes inside another.
'back' says the letters should be written in reverse.
'of ship' becomes 'munts' (I am not sure about this - if you are sure you should believe this answer much more).
'munts' back-to-front is 'stnum'.
'er' inserted within 'stnum' is 'STERNUM'.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "Munster (anag.)" , "big bone" , "somewhere in chest" , "Human bone -- ten rums (anag)" , "front of breast" .)