Complicated birth involving oxygen and end of term clots (7)
I believe the answer is:
thrombi
'clots' is the definition.
(thrombus is a kind of clot)
'complicated birth involving oxygen and end of term' is the wordplay.
'complicated' is an anagram indicator (a complex or confusing ordering of the letters).
'involving' is an insertion indicator.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'and' is a charade indicator (letters next to each other).
'end of' says to take the final letters.
The last letter of 'term' is 'm'.
'birth' with letters rearranged gives 'thrbi'.
'o'+'m'='om'
'thrbi' going around 'om' is 'THROMBI'.
(Another definition for thrombi that I've seen is " Blood clots".)