Taking in oxygen, increasingly difficult for squirrel (7)
I believe the answer is:
hoarder
'for squirrel' is the definition.
(I've seen this before)
'taking in oxygen increasingly difficult' is the wordplay.
'taking in' indicates putting letters inside.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'increasingly difficult' becomes 'harder'.
'o' placed into 'harder' is 'HOARDER'.
(Other definitions for hoarder that I've seen before include "One secretly stores money or valuables" , "Accumulator" , "Collector" , "One gathers" , "Person acting like a squirrel" .)