Sets in, still involved with the vessel (7)

'sets' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

'in still involved with the vessel' is the wordplay.
'still' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'involved' means one lot of letters goes inside another.
'with' means one lot of letters goes inside another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
't' inserted inside 'ss' is 'sts'.
'er' put inside 'sts' is 'serts'.
'in'+'serts'='INSERTS'

Can you help me to learn more?