Splinter is small and solid (5)
I believe the answer is:
shard
'splinter' is the definition.
(thesaurus)
'small and solid' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'solid' becomes 'hard' (associated in meaning).
's'+'hard'='SHARD'
'is' acts as a link.
(Other definitions for shard that I've seen before include "construction in London" , "Sliver of glass, etc" , "Bit of pottery" , "Broken piece of a brittle artefact" , "remnant after break-up" .)