Having constant F pitch is not that important (8)
I believe the answer is:
piffling
'having' is the definition.
The definition and answer can be both related to bodily activities as well as being present participle verbs.
Maybe you can see an association between them that I don't see?
'constant f pitch is not that important' is the wordplay.
I cannot quite see how this works, but
'constant' could be 'pi' (mathematics: 3.14) and 'pi' is found within the answer.
'f' is found in the answer.
'pitch' could be 'fling' and 'fling' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for piffling that I've seen before include "Informally trivial or worthless" , "Trivially tiny" , "Mickey Mouse" , "Of no importance at all" , "Minor" .)