Tense boast of bighead? (9)
I believe the answer is:
imperfect
'tense boast of bighead?' is the definition.
'imperfect' can be an answer for 'tense' (the imperfect tense is a grammatical tense). I am unsure of the 'boast of bighead?' bit.
'tense' is the wordplay.
I cannot quite understand how this works, but
'tense' could be 'perfect' (perfect tense is a kind of tense) and 'perfect' is present in the answer.
The remaining letters 'im' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for imperfect that I've seen before include "Having faults" , "Deficient" , "Flawed, defective" , "Faulty; tense" , "seconds" .)