Gutless idiot's bare bum (4)
I believe the answer is:
moon
'bare bum' is the definition.
'gutless idiot's' is the wordplay.
'gutless' says to hollow out the word (remove centre letters).
'idiot' becomes 'moron' (synonyms).
'moron' with its middle removed is 'MOON'.
(Other definitions for moon that I've seen before include "Heavenly body < exemplified in Titania?" , "show a lot of cheek" , "Phoebe" , "Earthly satellite" , "The Earth's natural satellite" .)