Rugby player runs into dad (4)
I believe the answer is:
prop
'rugby player' is the definition.
(type of rugby player)
'runs into dad' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'dad' becomes 'pop' (synonyms).
'r' placed within 'pop' is 'PROP'.
(Other definitions for prop that I've seen before include "A rigid supprot or piece of satge furniture (4)" , "One is usually among the strongest and heaviest rugby players" , "one playing Rugby" , "Lean" , "Back up" .)