Pipe down keel's centre runs straight down (5)
I believe the answer is:
sheer
'straight down' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'pipe down keel's centre runs' is the wordplay.
'pipe down' becomes 'sh' (both can mean 'be quiet').
'centre' says to take the centre.
'runs' becomes 'r' (cricket abbreviation).
The middle letters of 'keel' are 'ee'.
'sh'+'ee'+'r'='SHEER'
Can you help me to learn more?
(Other definitions for sheer that I've seen before include "Steep; fine" , "Precipitous or fine" , "Nothing other than" , "Arrant" , "Gossamer or precipitous" .)