That lady's son trained birds (6)
I believe the answer is:
herons
'birds' is the definition.
(heron is a kind of bird)
'that lady's son trained' is the wordplay.
'that lady' becomes 'her' ('her' refers to a woman).
'trained' indicates an anagram (to train can mean to grow something into a shape).
'son' with letters rearranged gives 'ons'.
'her'+'ons'='HERONS'
(Other definitions for herons that I've seen before include "leggy birds" , "Wading birds with long legs" , "Waders" .)