To be sparing is of no avail (7)
I believe the answer is:
useless
'to be sparing' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
'of no avail' is the wordplay.
I cannot really see how this works, but
'avail' could be 'use' (I've seen this in another clue) and 'use' is found in the answer.
The remaining letters 'less' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for useless that I've seen before include "Not worth a jot" , "to no avail" , "Incompetent" , "Is Sue less impractical?" , "Far from impressive" .)