Puzzles son with promise to split burden (10)

'puzzles' is the definition.
(crossword is a type of puzzle)

'son with promise to split burden' is the wordplay.
'son' becomes 's'.
'with' means one lot of letters go next to another.
'promise' becomes 'word' (as in 'I give you my word').
'to split' means one lot of letters goes inside another.
'burden' becomes 'cross' (someone's particular burden is their cross).
's'+'word'='sword'
'sword' put inside 'cross' is 'CROSSWORDS'.