Assignment while filling empty truck (4)
I believe the answer is:
task
'assignment' is the definition.
(I've seen this in another clue)
'while filling empty truck' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'filling' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'truck' with its middle taken out is 'tk'.
'as' going inside 'tk' is 'TASK'.
(Other definitions for task that I've seen before include "Chore, job." , "Piece of work to be done" , "Job to be done" , "Difficult piece of work" , "Give mission" .)