Come out -- father's climbing tree! (6)
I believe the answer is:
appear
'come out' is the definition.
(synonyms)
'father's climbing tree' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'tree' becomes 'pear' (pear tree is a kind of tree).
'pa' back-to-front is 'ap'.
'ap'+'pear'='APPEAR'
(Other definitions for appear that I've seen before include "Perform" , "be published" , "Seem - come into view" , "Come into sight, materialise" , "Develop" .)