Sustain hem tailoring in keenness (10)
I believe the answer is:
enthusiasm
'keenness' is the definition.
(thesaurus)
'sustain hem tailoring' is the wordplay.
'tailoring' indicates anagramming the letters.
'sustain'+'hem'='sustainhem'
'sustainhem' anagrammed gives 'ENTHUSIASM'.
'in' acts as a link.
(Other definitions for enthusiasm that I've seen before include "Intense and lively interest" , "Avidity" , "Ardour" , "Gusto; fervour" , "spirit?" .)