Satisfied the requirement of some transport line (3)
I believe the answer is:
met
'satisfied the requirement' is the definition.
'met' can be an answer for 'satisfied' (as in 'met the conditions of his contract'). I am not certain of the 'the requirement' bit.
'of some transport line' is the wordplay.
'of' says the answer is hidden in the clue.
'MET' is hidden within 'some transport line'.
Can you help me to learn more?
(Other definitions for met that I've seen before include "Encountered, came into contact" , "Bumped into" , "Opera company" , "Encountered, came across" , "Came together, encountered" .)