Fare-dodger observed commandeering tug, certainly (8)
I believe the answer is:
stowaway
'fare-dodger' is the definition.
(I've seen this before)
'observed commandeering tug certainly' is the wordplay.
'observed' becomes 'saw' (seen).
'commandeering' means one lot of letters goes inside another.
'tug' becomes 'tow' (I've seen this in another clue).
'certainly' becomes 'ay' (variant spelling of 'aye').
'saw' placed around 'tow' is 'stowaw'.
'stowaw'+'ay'='STOWAWAY'
(Other definitions for stowaway that I've seen before include "Illegal ship's passenger" , "Person who hides on ship to avoid paying fare" , "Free rider" , "Store - secret passenger" , "pack" .)