Genuine Dad crossing this country park finally (5)
I believe the answer is:
pukka
'genuine' is the definition.
(thesaurus)
'dad crossing this country park finally' is the wordplay.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'crossing' is an insertion indicator.
'this country' becomes 'uk' (from the perspective of a British newspaper).
'finally' indicates one should take the final letters.
The final letter of 'park' is 'k'.
'uk'+'k'='ukk'
'pa' enclosing 'ukk' is 'PUKKA'.
(Other definitions for pukka that I've seen before include "Genuine; reliable" , "High-class - permanent" , "Genuine, proper" , "not mock" , "that's for sure" .)