Bad-tempered worker beset by ulcer, old American (12)
I believe the answer is:
cantankerous
'bad-tempered' is the definition.
(thesaurus)
'worker beset by ulcer old american' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'ulcer' becomes 'canker' (canker is a kind of ulcer).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us'.
'ant' placed into 'canker' is 'cantanker'.
'cantanker'+'o'+'us'='CANTANKEROUS'
(Other definitions for cantankerous that I've seen before include "Disagreeable" , "Awkward" , "can be prickly, too!" , "like a wrangler" , "Contrary and quarrelsome" .)