Half-hearted “yes” from Ruth, thought to be oddly absent (2-3)
I believe the answer is:
uh-huh
'half-hearted yes' is the definition.
(answer expressing some but not total agreement)
'ruth thought to be oddly absent' is the wordplay.
'to be oddly absent' means one should take alternating letters (odd-numbered letters removed).
'ruth'+'thought'='ruththought'
The alternating letters of 'ruththought' are 'UH-HUH'.
'from' acts as a link.
(Other definitions for uh-huh that I've seen before include "OK! (interjection)" , ""Yes" interjection" , "Yup!" .)