Tammy's out of turn, but may do a good deed (5)
I believe the answer is:
scout
'may do a good deed' is the definition.
I can't judge whether this definition defines the answer.
'tammy's out of turn' is the wordplay.
'tammy' becomes 'scot' (I can't justify this - if you can you should give a lot more credence to this answer).
'out of' means one lot of letters goes inside another.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'scot' enclosing 'u' is 'SCOUT'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for scout that I've seen before include "Information-gathering soldier" , "One on a recce mission" , "Soldier on a recce mission" , "Talent seeker - boy on a jamboree" , "Person sent out to gain information" .)