Lock in top scientific assessment in system monitoring (6,4)

'system monitoring' is the definition.
(a test testing a certain system)

'lock in top scientific assessment' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'in' means one lot of letters goes inside another.
'top' says to take the initial letters.
'assessment' becomes 'test' (test is a kind of assessment).
The first letter of 'scientific' is 's'.
's'+'test'='stest'
'tress' inserted into 'stest' is 'STRESS TEST'.

'in' is the link.