Thin pipe and fork (5)
I believe the answer is:
reedy
'thin' is the definition.
(as in a thin or reedy voice)
'pipe and fork' is the wordplay.
'pipe' becomes 'reed pipe' (reed pipe is a kind of pipe).
'and' means one lot of letters go next to another.
'fork' becomes 'y' (I can't explain this - if you can you should believe this answer much more).
'reed'+'y'='REEDY'
Can you help me to learn more?
(Other definitions for reedy that I've seen before include "abounding with grasses" , "(Of a person) tall and thin" , "(Of voice) high/thin" , "That'll sound a bit thin" , "(Of sound) thin in tone" .)