Dad bags country house's top fish (7)
I believe the answer is:
piranha
'fish' is the definition.
(piranha is a kind of fish)
'dad bags country house's top' is the wordplay.
'dad' becomes 'pa' (synonyms).
'bags' means one lot of letters goes inside another (bag can mean secures or steals).
'country' becomes 'iran' (Iran is an example).
'top' says to take the initial letters.
The first letter of 'houses' is 'h'.
'iran'+'h'='iranh'
'pa' enclosing 'iranh' is 'PIRANHA'.
(Other definitions for piranha that I've seen before include "fish chewed up" , "Efficient flesh remover" , "Small voracious carnivorous fish" , "Voracious eater" , "Sharp-toothed fish" .)