After onset of sciatica, frequently notice weakness (4,4)
I believe the answer is:
soft spot
'weakness' is the definition.
(thesaurus)
'after onset of sciatica frequently notice' is the wordplay.
'after' is a charade indicator (letters next to each other).
'onset of' suggests taking the first letters.
'frequently' becomes 'oft' (I've seen this before).
'notice' becomes 'spot' (I've seen this before**).
The initial letter of 'sciatica' is 's'.
's'+'oft' is 'soft'.
'soft'+'spot'='SOFT SPOT'
(Other definitions for soft spot that I've seen before include "Emotional susceptibility" , "Feeling of affection towards person - not hard" , "Sentimental fondness" , "Affectionate weakness for (someone/something)" , "showing fondness" .)