Central London transport? It's subject to inflation (5,4)
I believe the answer is:
inner tube
'subject to inflation' is the definition.
I can't judge whether this definition defines the answer.
'central london transport?' is the wordplay.
'central' becomes 'inner' (similar in meaning).
'london transport?' becomes 'tube' (I've seen this in another clue).
'inner'+'tube'='INNER TUBE'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for inner tube that I've seen before include "What cyclist pumps up" , "One used to inflation going by bicycle" , "inflatable ring" , "Circle line?" , "Tyre filler" .)