Mathematical functions with singular snug quality (8)
I believe the answer is:
cosiness
'snug quality' is the definition.
(I've seen this before)
'mathematical functions with singular' is the wordplay.
'mathematical functions' becomes 'cosines' (cosine is a kind of mathematical function).
'with' says to put letters next to each other.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'cosines'+'s'='COSINESS'
(Other definitions for cosiness that I've seen before include "comfortable feeling" , "Snug quality" , "Intimacy" , "Comfort, warmth" , "Warm quality" .)