Bodysuit won’t best shield guts (6)
I believe the answer is:
onesie
'bodysuit' is the definition.
(I know that bodysuit can be written as onesie)
'best shield guts' is the wordplay.
'best' becomes 'ones' (I can't explain this - if you can you should believe this answer much more).
'guts' means to look at the middle letters (the insides of the word).
The middle letters of 'shield' are 'ie'.
'ones'+'ie'='ONESIE'
'won't' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for onesie that I've seen before include "One-piece full-body garment" , "Single-piece full body garment" .)