Weariness shown by girl initially unwavering in new diet (9)
I believe the answer is:
lassitude
'weariness' is the definition.
(I've seen this before)
'girl initially unwavering in new diet' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'initially' indicates taking the first letters.
'in' indicates putting letters inside.
'new' indicates anagramming the letters.
The initial letter of 'unwavering' is 'u'.
'diet' anagrammed gives 'itde'.
'u' going inside 'itde' is 'itude'.
'lass'+'itude'='LASSITUDE'
'shown by' is the link.
(Other definitions for lassitude that I've seen before include "Listlessness, state of apathy" , "30 [LETHARGY]" , "Weariness" , "Is saluted with tiredness" , "Weakness" .)