Sheep tucking into pig's food could be a life-saver (3,4)
I believe the answer is:
mae west
'a life-saver' is the definition.
(Mae West is an informal name for a life jacket)
'sheep tucking into pig's food' is the wordplay.
'sheep' becomes 'ewes' (a ewe is a female sheep).
'tucking into' indicates putting letters inside.
'pig's food' becomes 'mat' (I am not sure about this - if you are sure you should believe this answer much more).
'ewes' put within 'mat' is 'MAE WEST'.
'could be' acts as a link.
Can you help me to learn more?
(Other definitions for mae west that I've seen before include "embraced many men in the main" , "Actress - life jacket" , "Life jacket; film star" , "buoyant supporter" , "life-saver" .)