Pipe down keel's centre runs straight down (5)
I believe the answer is:
sheer
'straight down' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way 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 central 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" .)