Henry as well as Mike gets a shot in the arm from old servant (8)
I believe the answer is:
handmaid
'old servant' is the definition.
'handmaid' can be an answer for 'servant' (handmaid is a kind of servant). I am not certain of the 'old' bit.
'henry as well as mike gets a shot in the arm' is the wordplay.
'henry' becomes 'h' ('H' can be a synonym of 'henry').
'as well as' becomes 'and' (I've seen this in other clues).
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'gets' says to put letters next to each other.
'a shot in the arm' becomes 'aid' (I can't justify this - if you can you should believe this answer much more).
'h'+'and'+'m'+'aid'='HANDMAID'
'from' is the link.
Can you help me to learn more?
(Other definitions for handmaid that I've seen before include "Female attendant" , "servant - with 2d" , "Female servant" .)