One line crossed by old farmers, tough old birds (7)
I believe the answer is:
boilers
'tough old birds' is the definition.
(birds which must be cooked through boiling)
'one line crossed by old farmers' is the wordplay.
'one' becomes 'I' (Roman numeral).
'line' becomes 'l'.
'crossed by' is an insertion indicator (inserted letters are crossed or covered by others).
'old farmers' becomes 'Boers' (Dutch colonialists - 'boer' is farmer in Dutch).
'i'+'l'='il'
'il' placed within 'boers' is 'BOILERS'.
(Another definition for boilers that I've seen is " old birds".)