Small, rigid piece of glass? (5)
I believe the answer is:
shard
'piece of glass?' is the definition.
'small rigid' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'rigid' becomes 'hard' (I've seen this in another clue).
's'+'hard'='SHARD'
(Other definitions for shard that I've seen before include "chip" , "Broken piece (of pottery, glass etc)" , "Bit of pottery" , "Broken fragment" , "Pottery fragment" .)