Somehow mastered deal on pressure vessel (6,7)
I believe the answer is:
paddlesteamer
'vessel' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'somehow mastered deal on pressure' is the subsidiary indication.
'somehow' is an anagram indicator.
'on' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'mastered'+'deal'='mastereddeal'
'mastereddeal' anagrammed gives 'addlesteamer'.
'addlesteamer' after 'p' is 'paddlesteamer'.
Can you help me to learn more?
(Other definitions for paddlesteamer that I've seen before include "Wheel-driven boat" , "Wheel-driven vessel" , "Ship propelled by bladed wheels" , "Boat driven by wheels" , "Typical Mississippi vessel" .)