Grapple with first of lines in child's play (6)
I believe the answer is:
clinch
'grapple' is the definition.
(thesaurus)
'first of lines in child's play' is the wordplay.
'first of lines' becomes 'l' (1st letter of 'lines').
'in' means one lot of letters goes inside another.
'child's play' becomes 'cinch' (synonyms).
'l' going into 'cinch' is 'CLINCH'.
'with' is the link.
(Other definitions for clinch that I've seen before include "Fierce embrace" , "Confirm - embrace" , "Passionate embrace" , "Settle conclusively with informal embrace" , "Matthew ---; --- Schoenberg" .)