That time when a Yankee followed mum (3)
I believe the answer is:
may
'that time' is the definition.
(I've seen this before)
'a yankee followed mum' is the wordplay.
'a yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'followed' says to put letters next to each other.
'mum' becomes 'ma' (ma can mean mother).
'y' after 'ma' is 'MAY'.
'when' is the link.
(Other definitions for may that I've seen before include "plant that's tropical making comeback" , "'5 _____' (popular piece of music by Bach)" , "Spring" , "The permissive month" , "UK PM" .)