Compact and hard in a moment (5)
I believe the answer is:
thick
'compact' is the definition.
(I know that compact can be written as thick)
'hard in a moment' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in' indicates putting letters inside.
'a moment' becomes 'tick' (both can mean a very short amount of time).
'h' put within 'tick' is 'THICK'.
'and' is the link.
(Other definitions for thick that I've seen before include "Broad; dense" , "having such a head?" , "With relatively large distance between opposite sides" , "Stupid; (of voice) husky" , "unintelligent?" .)