Faithful worker goes after jailbird on street (8)
I believe the answer is:
constant
'faithful' is the definition.
('constant' can be similar in meaning to 'faithful')
'worker goes after jailbird on street' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'goes after' means one lot of letters go next to another.
'jailbird' becomes 'con' (I've seen this in other clues).
'on street' becomes 'st' (I've seen this in other clues).
'con'+'st'='const'
'ant' put after 'const' is 'CONSTANT'.
(Other definitions for constant that I've seen before include "Stable" , "Unwavering" , "Continuing unvarying" , "Faithful - unchanging" , "Steadfast or unvarying" .)