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'm not certain of the 'the requirement' bit.
'of some transport line' is the wordplay.
'of' indicates the answer is hidden within the clue.
'MET' is hidden in the letters of '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" .)