Small pub defending step which keeps characters apart (5,3)
I believe the answer is:
space bar
'which keeps characters apart' is the definition.
(the space bar introduces spaces to text)
'small pub defending step' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pub' becomes 'bar' (both are drinking establishments).
'defending' indicates putting letters inside.
'step' becomes 'pace' (pace is a kind of step).
's'+'bar'='sbar'
'sbar' going around 'pace' is 'SPACE-BAR'.
(Other definitions for space bar that I've seen before include "It is found on a keyboard" , "Restaurant at the End of the Universe?" , "Long key on a typewriter" , "used by typists?" , "It's on the bottom row of a keyboard" .)