Have to agree! 'Sense of loss' is clearly 'regret' but it's this that 'drops' its 'crest(top)' leaving the bird 'egret'.
Unusual as Paul is normally very precise with his clueing I think but don't see how it can be anything else.
I did toy with 'Eaglet' dropping 'E' to give 'aglet' but this means something completely different to 'sense of loss'.