Shut up! The fellow has a gun (6)
I believe the answer is:
'shut' is the definition.
(I've seen this before)
'up the fellow has a gun' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'the fellow' becomes 'f' (abbreviation - of a society etc.).
'has' means one lot of letters go next to another.
'gun' becomes 'sten' (British sub-machine gun).
'f' after 'a' is 'af'.
'af' written backwards gives 'fa'.
(Other definitions for fasten that I've seen before include "Fix securely" , "Attach" , "Fix to, secure" , "Tie, secure" , "Affix" .)