Walk about inside empty tank (4)
I believe the answer is:
trek
'walk' is the definition.
(I've seen this before)
'about inside empty tank' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'inside' is an insertion indicator.
'empty' says to hollow out the word (remove centre letters).
'tank' with its centre removed is 'tk'.
're' put inside 'tk' is 'TREK'.
(Other definitions for trek that I've seen before include "Long walk, expedition" , "Tramp" , "A long arduous march" , "A long arduous journey as in S Africa" , "Extended journey" .)