Cross, father swallows key (7)
I believe the answer is:
saltire
'cross' is the definition.
(saltire is a kind of cross)
'father swallows key' is the wordplay.
'father' becomes 'sire' (synonyms).
'swallows' indicates putting letters inside.
'key' becomes 'alt' (I've seen this in other clues).
'sire' going around 'alt' is 'SALTIRE'.
(Other definitions for saltire that I've seen before include "Heraldic cross" , "X-shaped cross" , "Diagonal cross (heraldry)" , "Scottish cross" , "flag - 8's? [SAINT ANDREW]" .)