While going round the National Theatre, they crawl! (4)
I believe the answer is:
ants
'they crawl' is the definition.
Although both the answer and 'crawl' are nouns, they are not in the same form of the noun.
'while going round the national theatre' is the wordplay.
'while' becomes 'as'.
'going round' indicates putting letters inside.
'the national theatre' becomes 'nt' (this might be a standard abbreviation which I don't know about).
'as' enclosing 'nt' is 'ANTS'.
Can you help me to learn more?
(Other definitions for ants that I've seen before include "Busy little insects" , "24 [INSECT] s" , "Inhabitants of a formicary" , "Termites" , "Tiny insects that live in colonies" .)