Little bit odd in empty catacomb (5)
I believe the answer is:
crumb
'little bit' is the definition.
'odd in empty catacomb' is the wordplay.
'odd' becomes 'rum' (I've seen this before).
'in' indicates putting letters inside.
'empty' suggests removing the centre.
'catacomb' with its middle removed is 'cb'.
'rum' placed into 'cb' is 'CRUMB'.
(Other definitions for crumb that I've seen before include "Just a little bit" , "of the farmhouse?" , "Tiny piece of bread or cake" , "A shred of bread" , "Small particle of bread" .)