Short old cock gutted in farmyard enclosure (6)
I believe the answer is:
stocky
'short' is the definition.
('stocky' can be similar in meaning to 'short')
'old cock gutted in farmyard enclosure' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gutted' suggests removing the centre (to gut something is to remove its insides).
'in' is an insertion indicator.
'farmyard enclosure' becomes 'sty' (I've seen this before).
'cock' with its middle removed is 'ck'.
'o'+'ck'='ock'
'ock' inserted within 'sty' is 'STOCKY'.
(Other definitions for stocky that I've seen before include "Strongly-built" , "Short and solidly built" , "Of short and solid stature" , "Short but well-built" , "Heavily-built" .)