They've been on the staff a long time and are experienced (3,5)
I believe the answer is:
old hands
'they've been on the staff a long' is the definition.
I can't tell whether this definition defines the answer.
'time and are experienced' is the wordplay.
'time' becomes 'old' (old is a kind of time).
'and' is a charade indicator (letters next to each other).
'are experienced' becomes 'hands' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'old'+'hands'='OLD HANDS'
Can you help me to learn more?
(Other definitions for old hands that I've seen before include "experienced types" , "arthritic strugglers when pen has to be put to paper?" , "Experts" , "Experienced or practised workers (3,5)" , "Experienced folk" .)