Terminus very large to manage - this allows an internal review (9)
I believe the answer is:
endoscope
'this allows an internal review' is the definition.
(instrument used in an endoscopy)
'terminus very large to manage' is the wordplay.
'terminus' becomes 'end' (eg the end of a train line is its terminus).
'very large' becomes 'os' (abbreviation for oversize).
'to manage' becomes 'cope' ('cope' can be a synonym of 'manage').
'end'+'os'+'cope'='ENDOSCOPE'
(Other definitions for endoscope that I've seen before include "Instrument for viewing internal body parts" , "medical probe" , "Medical instrument" , "Unwelcome camera" , "medical device" .)