Childish? Not half, in river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'childish? not half' is the wordplay.
'childish?' becomes 'juvenile' (I've seen this before).
'not half' means to take half the letters.
'juvenile' with half the letters taken is 'NILE'.
'in' acts as a link.
(Other definitions for nile that I've seen before include "eastern flower" , "Long river" , "long-distance runner in Africa" , "Cairo's river" , "White or Blue river of Africa" .)