Poky numismatist’s centre with everything under sun (5)
I believe the answer is:
small
'poky' is the definition.
(thesaurus)
'numismatist's centre with everything under sun' is the wordplay.
'centre' indicates the central letters.
'with' says to put letters next to each other.
'everything' becomes 'all' (all things).
'under' says to put letters next to each other (below, in a down clue).
'sun' becomes 'S' (abbreviation).
The middle of 'numismatist' is 'm'.
'm'+'all' is 'mall'.
'mall' put after 's' is 'SMALL'.
(Other definitions for small that I've seen before include "Insignificant; young" , "Minute" , "Undersized" , "Diminutive" , "Unimportant; little" .)