Prison ship getting old American irritable (12)
I believe the answer is:
cantankerous
'irritable' is the definition.
(I've seen this before)
'prison ship getting old american' is the wordplay.
'prison' becomes 'can' (the can is American slang for prison).
'ship' becomes 'tanker' (ship carrying liquids).
'getting' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'US' (abbreviation for United States).
'can'+'tanker'+'o'+'us'='CANTANKEROUS'
(Other definitions for cantankerous that I've seen before include "Grouchy" , "Bloody-minded -- non-UK car seat (anag)" , "Awkward" , "Stubbornly obstructive" , "Grumpy" .)