Straggle headed by elderly goose (7)
I believe the answer is:
greylag
'goose' is the definition.
(greylag goose is a kind of goose)
'straggle headed by elderly' is the wordplay.
'straggle' becomes 'lag' (I've seen this in another clue).
'headed by' means one lot of letters go next to another.
'elderly' becomes 'grey' (I've seen this in other clues).
'lag' after 'grey' is 'GREYLAG'.
(Other definitions for greylag that I've seen before include "bird?" , "Large goose" , "Eurasian goose" , "Breed of goose - Lyra egg (anag)" , "Wild goose -- eg gyral (anag)" .)