Promise without really delivering, at first (4)
I believe the answer is:
word
'promise' is the definition.
(as in 'I give you my word')
'without really delivering at first' is the wordplay.
'without' becomes 'wo' (abbreviation of 'without', normally with a slash).
'at first' indicates taking the first letters.
The initial letters of 'really delivering' is 'rd'.
'wo'+'rd'='WORD'
(Other definitions for word that I've seen before include "See 1" , "usually comes with more than one letter" , "Term" , "News; speech element" , "Remark; news" .)