Child was portrayed by this actress when about 60 (6)
I believe the answer is:
streep
'child was portrayed by this actress' is the definition.
'streep' can be an answer for 'actress' (I've seen this before). I am not certain of the remainder of the definition.
'about 60' is the wordplay.
I cannot quite see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found in the answer.
The remaining letters 'step' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'when' acts as a link.
(Other definitions for streep that I've seen before include "Inflamed swelling inside" , "Meryl --, Oscar winner" , "[Thespian]" , "Film star in The Iron Lady, b. 1949" , "Meryl -, US actress" .)