Murphy retired, developing an insecticide (9)

'an insecticide' is the definition.
(I know that pyrethrum is a type of insecticide)

'murphy retired' is the wordplay.
I cannot really understand how this works, but
'retired' could be 'ret' (e.g. following the name of a retired army officer) and 'ret' is found in the answer.
an anagram of 'murphy' is 'pyhrum' which is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

'developing' acts as a link.

Can you help me to learn more?