Engineers inside still moving lattice (7)
I believe the answer is:
trellis
'lattice' is the definition.
(I know that trellis is a type of lattice)
'engineers inside still moving' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'inside' means one lot of letters goes inside another.
'moving' indicates anagramming the letters.
'still' is an anagram of 'tllis'.
're' placed within 'tllis' is 'TRELLIS'.
(Other definitions for trellis that I've seen before include "Latticework for climbing plants" , "trainer" , "Support for creepers" , "latticework in the garden" , "Latticework support for plants" .)