Blue, having cut short dive — lacking this? (8)
I believe the answer is:
aqualung
'lacking this?' is the definition.
The definition suggests a singular noun which matches the answer.
'blue having cut short dive' is the wordplay.
'blue' becomes 'aqua' (aqua is a kind of blue).
'having' means one lot of letters go next to another.
'cut short dive' becomes 'lung' (I can't explain this - if you can you should give a lot more credence to this answer).
'aqua'+'lung'='AQUALUNG'
Can you help me to learn more?
(Other definitions for aqualung that I've seen before include "Diving apparatus" , "Diving equipment with cylinders of compressed air" , "Diver's breathing apparatus" , "Aid for diver" , "Underwater breathing apparatus" .)