Crack top of bath and rust develops (5)
I believe the answer is:
burst
'crack' is the definition.
(thesaurus)
'top of bath and rust develops' is the wordplay.
'top of' suggests taking the first letters.
'and' means one lot of letters go next to another.
'develops' is an anagram indicator.
The first letter of 'bath' is 'b'.
'rust' anagrammed gives 'urst'.
'b'+'urst'='BURST'
(Other definitions for burst that I've seen before include "Explosion" , "Fit" , "Split asunder" , "Explode, break open" , "Suddenly break open" .)