Grouchy old American chasing tin vessel (12)
I believe the answer is:
cantankerous
'grouchy' is the definition.
(thesaurus)
'old american chasing tin vessel' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us'.
'chasing' says to put letters next to each other.
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'vessel' becomes 'tanker' (tanker is a kind of vessel**).
'o'+'us'='ous'
'can'+'tanker'='cantanker'
'ous' after 'cantanker' is 'CANTANKEROUS'.
(Other definitions for cantankerous that I've seen before include "Difficult" , "Stubbornly obstructive" , "Grumpy" , "can be prickly, too!" , "Disagreeable" .)