Replacements may be sparse (6)
I believe the answer is:
spares
'replacements' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Perhaps there's a link between them I don't understand?
'may be sparse' is the wordplay.
'may be' is an anagram indicator.
'sparse' with letters rearranged gives 'SPARES'.
Can you help me to learn more?
(Other definitions for spares that I've seen before include "Is merciful towards, refrains from hurting" , "Lets off; reserve parts" , "Extras with spears" , "Replacement items, like tyres" , "Extras in case of necessity" .)