Test concentration of class descended on by singer (7)
I believe the answer is:
titrate
'test concentration' is the definition.
(chemistry term)
'class descended on by singer' is the wordplay.
'class' becomes 'rate' (class can mean to rank or rate).
'descended on by' says to put letters next to each other (in a down clue, some letters descend onto others).
'singer' becomes 'tit' (type of songbird).
'rate' after 'tit' is 'TITRATE'.
'of' acts as a link.
(Other definitions for titrate that I've seen before include "Use indicator maybe" , "Carry out scientific analysis" , "Experimental way to measure" , "Measure the strength of (a solution) using a reagent" , "measure volume" .)