Son reluctant to show sluggishness (5)
I believe the answer is:
sloth
'sluggishness' is the definition.
(thesaurus)
'son reluctant' is the wordplay.
'son' becomes 's'.
'reluctant' becomes 'loth' ('loth' can be a synonym of 'reluctant').
's'+'loth'='SLOTH'
'to show' acts as a link.
(Other definitions for sloth that I've seen before include "Indolence" , "Lethargy" , "Idleness" , "Sluggish creature" , "Deadly sin - upside-down mammal" .)