Small, unyielding splinter (5)
I believe the answer is:
shard
'splinter' is the definition.
(thesaurus)
'small unyielding' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'unyielding' becomes 'hard' (I've seen this before).
's'+'hard'='SHARD'
(Other definitions for shard that I've seen before include "Broken fragment" , "Scrap of pottery" , "Go well with; attire" , "Sharp piece of glass" , "Sliver of glass, etc" .)