How full eg trains are raised when a parliamentarian is involved (3-6)
I believe the answer is:
jam-packed
'how full eg trains are' is the definition.
(jam-packed means very full)
'raised when a parliamentarian is involved' is the wordplay.
'raised when' becomes 'jacked' (jacking is a kind of raising. I am not sure about the 'when' bit.).
'parliamentarian' becomes 'MP' (Member of Parliament).
'is involved' means one lot of letters goes inside another.
'a'+'mp'='amp'
'jacked' placed around 'amp' is 'JAM-PACKED'.
(Other definitions for jam-packed that I've seen before include "Chock-a-block" , "Extremely crowded" , "Stuffed" , "Completely full" , "Heaving" .)