Children's fantasy character is beautiful, young and, above all, also extremely tough (5,5)
I believe the answer is:
tooth fairy
'children's fantasy character' is the definition.
(the Tooth Fairy is an imaginary character)
'beautiful young and above all also extremely tough' is the wordplay.
'beautiful' becomes 'fair' (similar in meaning).
'young' becomes 'y'.
'and above all' means one lot of letters go next to another (in a down clue, some letters go above others).
'also' becomes 'too' (synonyms).
'extremely' suggests removing the centre.
'tough' with its centre taken out is 'th'.
'fair'+'y'='fairy'
'too'+'th'='tooth'
'fairy' after 'tooth' is 'TOOTH FAIRY'.
'is' acts as a link.
(Other definitions for tooth fairy that I've seen before include "Parent really" , "Visitor leaving kids money" , "It may bring comfort to a child" , "Spirit said to leave money at night in compensation for losing a gnasher" , "She leaves money for molar" .)