A growing need to put Ed in the reserves (5)
I believe the answer is:
reeds
'a growing need to' is the definition.
I can't tell whether this definition defines the answer.
'ed in the reserves' is the wordplay.
'in' indicates putting letters inside.
'the reserves' becomes 'res' (I am not sure about this - if you are sure you should believe this answer much more).
'ed' placed within 'res' is 'REEDS'.
'put' acts as a link.
Can you help me to learn more?
(Other definitions for reeds that I've seen before include "Music-makers" , "pens for sheep" , "Marsh plants with tall straight stems" , "Stems for pens" , "Woodwind instruments" .)