Assessed speed on start of descent (5)
I believe the answer is:
rated
'assessed' is the definition.
(I've seen this before)
'speed on start of descent' is the wordplay.
'speed' becomes 'rate' (rate is a kind of speed**).
'on' means one lot of letters go next to another.
'start of' says to take the initial letters.
The first letter of 'descent' is 'd'.
'rate'+'d'='RATED'
(Other definitions for rated that I've seen before include "Thought highly of" , "Estimated the value or assigned a ranking" , "Estimated, set at" , "deserved" , "Assessed; merited" .)