Underground target started as something inflatable (5,4)
I believe the answer is:
inner tube
'something inflatable' is the definition.
I can't tell whether this defines the answer.
'underground target started' is the wordplay.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'target' becomes 'inner' (I've seen this before).
'started' is a charade indicator (letters next to each other).
'tube' put after 'inner' is 'INNER TUBE'.
'as' is the link.
Can you help me to learn more?
(Other definitions for inner tube that I've seen before include "inflatable ring" , "Circle line?" , "One used to inflation going by bicycle" , "Tyre filler" , "rubber ring" .)