Invincible perhaps, informed on fellow prisoners (8)
I believe the answer is:
flagship
'invincible perhaps' is the definition.
I can't tell whether this definition defines the answer.
'informed on fellow prisoners' is the wordplay.
'informed' becomes 'hip' (similar in meaning).
'on' means one lot of letters go next to another.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'prisoners' becomes 'lags' (lag is slang for a convict).
'f'+'lags'='flags'
'hip' put after 'flags' is 'FLAGSHIP'.
Can you help me to learn more?
(Other definitions for flagship that I've seen before include "Victory, perhaps" , "Admiral's vessel" , "Pre-eminent item" , "Pre-eminent product" , "leading product" .)