Struggling near-dunce shows staying-power (9)
I believe the answer is:
endurance
'staying-power' is the definition.
'struggling near-dunce' is the wordplay.
'struggling' indicates an anagram (I've seen this in other clues).
'near'+'dunce'='neardunce'
'neardunce' anagrammed gives 'ENDURANCE'.
'shows' is the link.
(Other definitions for endurance that I've seen before include "Power to withstand hardship or stress" , "Lasting quality" , "Fortitude" , "Patient toleration" , "Shackleton took to the Antarctic" .)