Henry picking up old pet's whoopsie (2,4)
I believe the answer is:
oh dear
'whoopsie' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'henry picking up old pet's' is the wordplay.
I cannot really understand how this works, but
'henry' could be 'h' (symbol for the unit of electrical inductance) and 'h' is found in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
'pet' could be 'dear' (I've seen this in another clue) and 'dear' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oh dear that I've seen before include "What a pity!" , "I say" , "What a pity/shame/surprise!" , "Expression of dismay" , "That's a shame" .)