Pose price mistakenly for part of a submarine (9)
I believe the answer is:
periscope
'for part of a submarine' is the definition.
('periscope' is part of 'submarine')
'pose price mistakenly' is the wordplay.
'mistakenly' indicates an anagram.
'pose'+'price'='poseprice'
'poseprice' with letters rearranged gives 'PERISCOPE'.
(Other definitions for periscope that I've seen before include "overseer" , "Submarine viewing aid" , "Sub's eye?" , "Optical instrument" , "Viewing instrument in a submarine" .)