It is grasped in firm and prompt handling (6)
I believe the answer is:
nettle
'it is grasped in firm' is the definition.
The definition suggests a singular noun which matches the answer.
'and prompt handling' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'prompt handling' becomes 'ettle' (I can't explain this - if you can you should give a lot more credence to this answer).
'n'+'ettle'='NETTLE'
Can you help me to learn more?
(Other definitions for nettle that I've seen before include "Upset" , "Stinging plant; annoy" , "Plant with stinging hairs" , "Sting or annoy with the plant" , "sample of herbal tea?" .)