It links extremely healthy writer with hospital investment (6)
I believe the answer is:
hyphen
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'extremely healthy writer with hospital investment' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'writer' becomes 'pen' (something which writes).
'with' is an insertion indicator.
'hospital investment' becomes 'h' (I am not sure about this - if you are sure you should believe this answer much more).
'healthy' with its middle taken out is 'hy'.
'pen' going around 'h' is 'phen'.
'hy'+'phen'='HYPHEN'
'links' is the link.
Can you help me to learn more?
(Other definitions for hyphen that I've seen before include "A horizontal words-joiner" , "A horizontal word-joiner" , "Word-joining mark" , "Punctuation mark separating words" , "Stroke between syllables or words" .)