Speed in small pub is key (5,3)
I believe the answer is:
space bar
'is key' is the definition.
(I've seen this before)
'speed in small pub' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed**).
'in' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pub' becomes 'bar' (I've seen this before).
's'+'bar'='sbar'
'pace' going inside 'sbar' is 'SPACE-BAR'.
(Other definitions for space bar that I've seen before include "The biggest typewriter key" , "Keyboard key" , "It's on the bottom row of a keyboard" , "used by typists?" , "Long key on a typewriter" .)