Only cut speed can affect body fluid (5)
I believe the answer is:
lymph
'body fluid' is the definition.
(I know that lymph is a type of body fluid)
'only cut speed can affect' is the wordplay.
I cannot quite understand how this works, but
'speed' could be 'mph' (miles per hour) and 'mph' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lymph that I've seen before include "Body fluid" , "discharge through wounds" , "Colourless fluid with white blood cells" , "Vaccine" , "Fluid associated with a gland" .)