First of riders in a lease causes alarm (5)
I believe the answer is:
alert
'alarm' is the definition.
('alert' can be a synonym of 'alarm')
'first of riders in a lease' is the wordplay.
'first of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'lease' becomes 'let' ('let' can be a synonym of 'lease').
The initial letter of 'riders' is 'r'.
'a'+'let'='alet'
'r' put inside 'alet' is 'ALERT'.
'causes' acts as a link.
(Other definitions for alert that I've seen before include "In state of watchfulness" , "Intellectually active" , "Wide-awake and watchful" , "Awake, attentive" , "sharp-witted" .)