Undertake to crack an egg, sweetheart! (6)
I believe the answer is:
engage
'undertake' is the definition.
(thesaurus)
'crack an egg sweetheart' is the wordplay.
'crack' indicates anagramming the letters.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'an'+'egg'='anegg'
'anegg' with letters rearranged gives 'engag'.
'engag'+'e'='ENGAGE'
'to' is the link.
(Other definitions for engage that I've seen before include "Join" , "Morally committed to" , "attack" , "Employ or pledge" , "Occupy (interest)" .)