Old woman getting home finds key (4)
I believe the answer is:
main
'key' is the definition.
(I've seen this in other clues)
'old woman getting home' is the wordplay.
'old woman' becomes 'ma' (I've seen this in other clues).
'getting' means one lot of letters go next to another.
'home' becomes 'in' ('he's home' can mean 'he's in').
'ma'+'in'='MAIN'
'finds' acts as a link.
(Other definitions for main that I've seen before include "Chief - principal electricity cable" , "Principal sea" , "Most important or greatest in size" , "Chief; sea; pipe" , "Ocean or principal pipe" .)