Reserves bed with bit of extra support (9)
I believe the answer is:
subscribe
'support' is the definition.
(thesaurus)
'reserves bed with bit of extra' is the wordplay.
'reserves' becomes 'subs' (eg substitutes in football).
'bed with' becomes 'crib' (I've seen this before. I am not sure about the 'with' bit.).
'bit of' suggests taking the first letters.
The first letter of 'extra' is 'e'.
'subs'+'crib'+'e'='SUBSCRIBE'
(Other definitions for subscribe that I've seen before include "Receive or obtain regularly" , "Agree" , "Contribute money" , "Receive on a regular basis, magazine say" , "Put one's name down as a purchaser" .)