What rhymes with callapse?

Do you mean 'collapse'? If so, perhaps

'Perhaps'

'f-laps' ('flaps', broken, humorous rhyme, e.g. 'A crow, close to collapse/F-laps its wings....')

's-laps' (similar to above)

End-rhymes (last-syllable-only rhyme) could include:

baps (bread-rolls)

caps

chaps

gaps

Japs

laps/claps/flaps/slaps

maps

naps/snaps

paps

raps/craps/scraps/p'raps/traps/wraps

saps

taps

yaps

zaps