External layer red with comet disintegrating (8)
I believe the answer is:
ectoderm
'external layer' is the definition.
'ectoderm' can be an answer for 'layer' (ectoderm is a kind of layer). I am unsure of the 'external' bit.
'red with comet disintegrating' is the wordplay.
'with' says to put letters next to each other.
'disintegrating' indicates anagramming the letters (I've seen 'disintegrated' mean this).
'red'+'comet'='redcomet'
'redcomet' with letters rearranged gives 'ECTODERM'.
Can you help me to learn more?
(Other definitions for ectoderm that I've seen before include "outer layer" , "Early nervous stage" , "Embryonic layer" , "outer part" , "Embryo's outermost layer of cells" .)