Invalid entrance to nightclub gets jammed, not opening (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'entrance to nightclub gets jammed not opening' is the wordplay.
'entrance to' indicates taking the first letters.
'gets' says to put letters next to each other.
'jammed' becomes 'full' (synonyms).
'not opening' means to remove the first letter (the opening letter is removed).
The first letter of 'nightclub' is 'n'.
'full' with its first letter taken off is 'ull'.
'n'+'ull'='NULL'
(Other definitions for null that I've seen before include "Invalid, void" , "Amounting to nothing" , "Useless - nothing" , "Without force, validity" , "Legally void" .)