If struggling in cracking a nut, one must be so weak! (5)
I believe the answer is:
unfit
'weak' is the definition.
(thesaurus)
'if struggling in cracking a nut' is the wordplay.
'struggling' is an anagram indicator.
'in' is an insertion indicator.
'cracking' indicates an anagram.
'if' with letters rearranged gives 'fi'.
'nut' with letters rearranged gives 'unt'.
'fi' going within 'unt' is 'UNFIT'.
'one must be so' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for unfit that I've seen before include "In poor condition" , "In bad physical condition" , "Ill-suited" , "Not in good physical condition, or not suitable" , "Not prepared" .)